Method for making water-absorbing polymer particles having areas with inorganic solid particles and areas substantially free of inorganic solid particles

ABSTRACT

A method for making water-absorbing polymer particles is provided and includes a first aqueous polymerization solution including crosslinkers, polymerizable monomers and/or oligomers and inorganic solid particles, and a second aqueous solution including inorganic solid particles, crosslinkers and either polymerizable monomers and/or oligomers, or crosslinkable polymers. The average closest distance between two neighboring crosslinkers (R XL ) from the first aqueous polymerization solution or from the second aqueous solution for a specific X-load of the water-absorbing polymer particle is calculated via the formula below: 
     
       
         
           
             
               
                 
                   Rxl 
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                           ( 
                           
                             
                               1 
                               rho_dry 
                             
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                               x_L 
                               rho_liq 
                             
                           
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                             N 
                             A 
                           
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                               i 
                             
                              
                             
                               
                                 w_xl 
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                                 Mr_CSL 
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                       1 
                       3 
                     
                   
                 
               
               
                 
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                   I 
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FIELD OF THE INVENTION

The invention relates to a method for making water-absorbing polymer particles having areas with inorganic solid particles and areas substantially free of inorganic solid particles where the average closest distance between two crosslinkers for a specific X-load of the given water-absorbing polymer particle is calculated via a specific formula. The invention also relates to water-absorbing polymer particles obtained via the method herein and to an absorbent article comprising the water-absorbing polymer particles obtained via the method herein. The absorbent articles include, but are not limited to baby diapers, training pants, feminine hygiene sanitary pads and adult incontinence products.

BACKGROUND OF THE INVENTION

An important component of a disposable absorbent article such as a diaper is an absorbent core including water-absorbing polymer particles. These water-absorbing polymer particles ensure that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness. Especially useful water-absorbing polymer particles are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g. sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like.

These water-absorbing polymer particles need to have adequately high absorption capacity, as well as adequately high gel strength. Absorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. At the same time, the water-absorbing polymer particles need to have a good permeability for fluid transport through the swollen crosslinked polymers network.

The properties of water-absorbing polymer particles have been characterized in various ways. The absorbent capacity (CRC) in grams of liquid per gram of water-absorbing polymer particles has been used, as well as the absorption speed as measured by the Free Swell Rate (FSR) and their permeability as measured by the Urine Permeability Measurement (UPM) test.

Low molecular weight polymer chains that are not incorporated into the crosslinked polymer network exist in the water-absorbing polymer particles and are called “extractables”. These chains can be extracted from the crosslinked polymer network when the water-absorbing polymer particles are swollen in excess liquid. The “extractables” do not contribute to the water-absorbing polymer particles performance.

Moreover, the “extractables” increase the ionic strength in the environment of the water-absorbing polymer particles due to their charged groups and negatively impact the absorption capacity of the water-absorbing polymer particles. Therefore, the quantity of “extractables” is important in determining the optimum performance of the water-absorbing polymer particles. There is a need to provide water-absorbing polymer particles with a low quantity of “extractables”.

Water-absorbing polymer particles with relatively high permeability can be made by increasing the level of internal crosslinking or surface crosslinking, which increases the resistance of the swollen gel against deformation by an external pressure and decrease the number of “extractables”. However, increasing the level of internal crosslinking or surface crosslinking typically reduce the absorption capacity of the water-absorbing polymer particles.

On the contrary, decreasing the level of internal crosslinking or surface crosslinking in water-absorbing polymer particles leads to high absorption capacity of the water-absorbing polymer particles but also to a high number of “extractables” and a relatively low permeability.

Therefore, there is a need to provide water-absorbing polymer particles that presents a right balance between having high absorption capacity, high permeability and a low number of “extractables”.

It is desirable to find a method for making water-absorbing polymer particles with improved performances in terms of capacity and permeability while having a low quantity of “extractables”.

SUMMARY OF THE INVENTION

The present invention provides a method for making water-absorbing polymer particles by providing:

-   -   a first aqueous polymerization solution comprising crosslinkers,         polymerizable monomers and/or oligomers and inorganic solid         particles, wherein the weight ratio of inorganic solid particles         based on the dry solid content of the first aqueous         polymerization solution is from 0 to 0.5%,     -   a second aqueous solution comprising inorganic solid particles,         crosslinkers and either polymerizable monomers and/or oligomers,         or crosslinkable polymers wherein the weight ratio of inorganic         solid particles based on the dry solid content of the second         aqueous solution is more than 0.5%.

The average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution for a specific X-load of a water-absorbing polymer particle is calculated via the formula below:

$\begin{matrix} {{Rxl} = \left( \frac{\left( {\frac{1}{{rho}_{—}{dry}} + \frac{x_{—}L}{{rho}_{—}{liq}}} \right)}{N_{A} \cdot {\sum\limits_{i}\frac{w_{—}{xl}_{i}}{{Mr}_{—}{CXL}_{i}}}} \right)^{\frac{1}{3}}} & (I) \end{matrix}$

with x_L corresponding to the amount of liquid absorbed in the water-absorbing polymer particle in g _(liq)/g _(water-absorbing polymer particle),

rho_liq corresponding to the density at room temperature of the fluid that swells the water-absorbing polymer particle (generally saline of 0.9% w NaCl) in g/cm³,

rho_dry corresponding to the true density of the dry water-absorbing polymer particle in g/cm³,

Mr_CXL corresponding to the molar mass of the crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in g/mol,

w_xl corresponding to the weight ratio of crosslinkers in dry water-absorbing polymer particle from the first aqueous polymerization solution or from the second aqueous solution, respectively,

N_(A) corresponding to the Avogadro's number in mol⁻¹,

The average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle obtained via the formula above is at least as high as an average size of the inorganic solid particles or higher than an average size of the inorganic solid particles in the respective solution. The average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle is from 3 to 100 nm. The method comprises the steps of:

a) polymerizing the first aqueous polymerization solution to obtain precursor water-absorbing polymer particles,

b) mixing the precursor water-absorbing polymer particles with the second aqueous solution, and

c) polymerizing the mixed solution if the mixed solution comprises polymerizable monomers and/or oligomers or crosslinking the mixed solution if the mixed solution comprises crosslinkable polymers to obtain the water-absorbing polymer particles.

The invention also relates to the water-absorbing polymer particles obtained by the method according to the invention.

The invention also relates to an absorbent article comprising the water-absorbing polymer particles obtained via the method described herein.

The formula above determines the average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution for a specific X-load of the water-absorbing polymer particle in order to obtain water-absorbing polymer particles with good performances in terms of permeability and capacity and with a low quantity of “extractables”.

The average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle obtained via the formula above is at least as high as an average size of the inorganic solid particles or higher than an average size of the inorganic solid particles.

Therefore, the inorganic solid particles present an average range of size that can fit the average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle.

The inorganic solid particles that fit the average closest distance between two neighboring crosslinkers (R_(XL)) may act as barrier particles in the crosslinked polymer network such that the diffusion of the “extractables” outside of the crosslinked polymer network is limited but the influx of fluid through the crosslinked polymer network is not slowed down significantly by the inorganic solid particles.

The water-absorbing polymer particles obtained via the method of the invention may have a relatively low amount of “extractables”. The method of the invention may improve the permeability and the capacity of the water-absorbing polymer particles.

Therefore, the water-absorbing polymer particles show good performance properties. Especially, the surface-coated water-absorbing polymer particles may have a high permeability, a good absorption capacity and a good effective capacity.

Moreover, the inorganic solid particles may aggregate to form aggregates of inorganic solid particles. This may negatively impact the diffusion of fluid through the crosslinked polymer network of the water-absorbing polymer particles. Thus, the present method providing two different aqueous solutions with different amount of inorganic solid particles and comprising either two polymerization steps, or one polymerization step and one crosslinking step, enables to form water-absorbing polymer particles having one or more than one area(s) substantially free of inorganic solid particles. These water-absorbing polymer particles having this specific structure may improve the diffusion of fluid through the crosslinked polymer network.

Furthermore, having a lower quantity of inorganic solid particles is cost-efficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an exemplary absorbent article in the form of a diaper, which may comprise the water-absorbing polymer particles of the present invention, with some layers partially removed.

FIG. 2 is a transversal cross-section of the diaper of FIG. 1.

FIG. 3 is a top view of an exemplary absorbent article in the form of a diaper which may comprise the water-absorbing polymer particles of the present invention, with area(s) substantially free of absorbent material.

FIG. 4 is a transversal cross-section of the article of FIG. 3.

FIG. 5 is a transversal cross-section of the article taken at the same point as FIG. 4 where channels have formed in the core as a result of the diaper being loaded with fluid.

FIG. 6 is a partial cross-sectional side view of a suitable permeability measurement system for conducting the Urine Permeability Measurement Test.

FIG. 7 is a cross-sectional side view of a piston/cylinder assembly for use in conducting the Urine Permeability Measurement Test.

FIG. 8 is a top view of a piston head suitable for use in the piston/cylinder assembly shown in FIG. 7.

FIG. 9 is a cross-sectional side view of the piston/cylinder assembly of FIG. 7 placed on fritted disc for the swelling phase.

DETAILED DESCRIPTION OF THE INVENTION Definition

The term “Absorbent article” refers to devices that absorb and contain body exudates, and, more specifically, refers to devices that are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body. Absorbent articles may include diapers (baby diapers and diapers for adult incontinence), pants, inserts, feminine care absorbent articles such as sanitary napkins or pantiliners, and the like. The term “exudates” includes, but is not limited to, urine, blood, vaginal discharges, sweat and fecal matter. Preferred absorbent articles of the present invention are disposable absorbent articles, more preferably disposable diapers and disposable pants.

“Disposable” is used in its ordinary sense to mean an article that is disposed or discarded after a limited number of usage over varying lengths of time, for example, less than 20 usages, less than 10 usages, less than 5 usages, or less than 2 usages. If the disposable absorbent article is a diaper, a pant, sanitary napkin, sanitary pad or wet wipe for personal hygiene use, the disposable absorbent article is most often intended to be disposed after single use.

“Diaper” and “pant” refers to an absorbent article generally worn by babies, infants and incontinent persons about the lower torso so as to encircle the waist and legs of the wearer and that is specifically adapted to receive and contain urinary and fecal waste. In a pant, the longitudinal edges of the first and second waist region are attached to each other to a pre-form waist opening and leg openings. A pant is placed in position on the wearer by inserting the wearer's legs into the leg openings and sliding the pant absorbent article into position about the wearer's lower torso. A pant may be pre-formed by any suitable technique including, but not limited to, joining together portions of the absorbent article using refastenable and/or non-refastenable bonds (e.g., seam, weld, adhesive, cohesive bond, fastener, etc.). A pant may be preformed anywhere along the circumference of the article (e.g., side fastened, front waist fastened). In a diaper, the waist opening and leg openings are only formed when the diaper is applied onto a wearer by (releasably) attaching the longitudinal edges of the first and second waist region to each other on both sides by a suitable fastening system.

The term “true density” refers to the density of the water-absorbing polymer particle that make up a particulate solid in contrast to the bulk density, which measures the average density of a large volume of the water-absorbing polymer particle in a specific medium (usually air).

The term “size of the inorganic solid particle” refers to the largest dimension of an exfoliated inorganic solid particle such as clay platelet.

“Comprise,” “comprising,” and “comprises” are open ended terms, each specifies the presence of what follows, e.g., a component, but does not preclude the presence of other features, e.g., elements, steps, components known in the art, or disclosed herein. These terms based on the verb “comprise” should be read as encompassing the narrower terms “consisting of” which excludes any element, step, or ingredient not specified and “consisting essentially of” which limits the scope of an element to the specified materials or steps and those that do not materially affect the way the element performs its function. Any preferred or exemplary embodiments described below are not limiting the scope of the claims, unless specifically indicated to do so. The words “typically”, “normally”, “advantageously” and the likes also qualify elements which are not intended to limit the scope of the claims unless specifically indicated to do so.

Water-Absorbing Polymer Particles

“Water-absorbing polymers” or “Superabsorbent polymers” refer to absorbent material which are crosslinked polymeric materials that can absorb at least 10 times their weight of an aqueous 0.9% saline solution as measured using the Centrifuge Retention Capacity (CRC) test method (EDANA method NWSP 241.0.R2). These polymers are typically used in particulate forms (“Water-absorbing polymer particles” or “Superabsorbent polymers particles”) so as to be flowable in the dry state. The term “particles” refers to granules, fibers, flakes, spheres, powders, platelets and other shapes and forms known to persons skilled in the art of water-absorbing polymer particles.

Preferred water-absorbing polymer particles of the present invention are made of poly(meth)acrylic acid polymers. However, e.g. starch-based particulate absorbent polymer material may also be used, as well polyacrylamide copolymer, ethylene maleic anhydride copolymer, cross-linked carboxymethylcellulose, polyvinyl alcohol copolymers, cross-linked polyethylene oxide, and starch grafted copolymer of polyacrylonitrile.

Typically, the water-absorbing polymer particles comprise crosslinked polymers, preferably lightly crosslinked hydrophilic polymers. While these polymers may in general be non-ionic, cationic, zwitterionic or anionic, the preferred polymers are cationic or anionic.

Preferably, water-absorbing polymer particles comprise acid polymers which contain a multiplicity of acid functional groups such as carboxylic acid groups or their salts, preferably sodium salts.

Preferably, the water-absorbing polymer particles comprise crosslinked polymers of polyacrylic acids or their salts or polyacrylates or derivatives thereof.

Exemplary water-absorbing polymer particles of the prior art are for example described in WO2006/083584, WO2007/047598, WO2007/046052, WO2009/155265, WO2009/155264.

The water-absorbing polymer particles can be spherical shaped water-absorbing polymer particles or ellipsoidal shaped water-absorbing polymer particles or irregular shaped water-absorbing polymer particles. Preferably, the water-absorbing polymer particles have a spherical or ellipsoid shape.

Preferably water-absorbing polymer particles have a particle size distribution in the range from 45 μm to 850 μm, or more preferably from 100 μm to 850 μm or 150 μm to 710 μm or 150 μm to 500 μm or 150 μm to 300 μm as measured according to EDANA method WSP 220.2-05.

Inorganic Solid Particles

The water-absorbing polymer particles comprise inorganic solid particles.

Inorganic solids particles may be clay particles. The clay particles may be in the form of platelets, e.g. exfoliated or individual clay particles in the form of platelets, having a largest dimension and a smallest dimension. For example, the largest dimension to smallest dimension ratio may be at least 2:1 or at least 10:1 or at least 25:1, up to 200:1 or up to 500:1. Preferably, the inorganic solid particles are clay platelets.

Clay and Clay Platelets

The water-absorbing polymer particles may comprise clay platelets as inorganic solid particles. Clay platelets have edge surfaces also referred to as “edges” and opposing basal platelet surfaces also referred to as “surfaces”. The clay platelets may be surface and/or edge-modified.

The surface and/or edge-modified clay platelets may have a weight average largest particle size dimension (length) of less than 800 nm, preferably less than 500 nm, more preferably less than 300 nm, more preferably less than 200 nm, even more preferably less than 100 nm according to the use of a X-ray microscopy, for example, Xradia 810 Ultra 3D X-ray Microscope commercialized by Zeiss or which may be determined via removal of a micro-slice of the water-absorbing polymer particles (via a ultramicrotome) which is then subjected to a cryo-TEM methods, known in the art or by the use of the dynamic light scattering test method.

The dynamic light scattering test method is described in the article: Karpovich, A et al, “Determination of dimensions of exfoliating materials in aqueous suspensions”, MethodsX, 2016, 3, 19-24. NMR relaxometry test method may also be used and is described in the same article above.

The surface and/or edge-modified clay platelets in the solution may have a weight average largest particle size dimension (length) of at least 5 nm, preferably of at least 10 nm, more preferably of at least 20 nm according to the use of a X-ray microscopy, for example, Xradia 810 Ultra 3D X-ray Microscope commercialized by Zeiss or which may be determined via removal of a micro-slice of the water-absorbing polymer particles (via a ultramicrotome) which is then subjected to a cryo-TEM methods, known in the art or by the use of the dynamic light scattering test method.

When the clay platelets have a large size dimension, it may be beneficial to break the larger size clay platelets by using an ultrasonic treatment before assessing their weight average largest particle size dimension as described above.

Examples of suitable clay platelets are selected from the group consisting of kaolinite such as kaolin, illite such as glauconite, or smectite or montmorillonite including hectorite, laponite (i.e. synthetic clay), saponite, vermiculite or mixtures thereof.

Preferably, the clay platelets are montmorillonite, hectorite, laponite or mixtures thereof.

Preferably, the clay platelets are laponite.

The clay platelets with edge modification and/or surface modification have a sterically hindering moiety(s). This characteristic may hinder and reduce aggregation of clay platelets.

Area(s) Substantially Free of Inorganic Solid Particles and Area(s) with Inorganic Solid Particles

The water-absorbing polymer particles may comprise one, or more than one area(s) with inorganic solid particles and one, or more than one area(s) substantially free of inorganic solid particles.

“Substantially free of inorganic solid particles” is used herein to mean that the water-absorbing polymer particles comprising one or more than one area(s) substantially free of inorganic solid particles correspond to water-absorbing polymer particles comprising one or more than one area(s) having an unsubstantial amount of water-absorbing polymer particles that does not affect the performance properties of the water-absorbing polymer particles. This may also mean that the water-absorbing polymer particles comprise one or more than one area(s) with no inorganic solid particles.

The volume of the area(s) with inorganic solid particles in dry state may extend in a first direction of at least 3 μm and in a second direction of at least 3 μm, the first direction and the second direction being perpendicular to each other, which may be determined e.g. via removal of a micro-slice of the water-absorbing polymer particles (via a ultramicrotome) which is then subjected to a cryo-TEM methods, known in the art or by the use of an X-ray microscopy, for example, Xradia 810 Ultra 3D X-ray Microscope commercialized by Zeiss.

The volume of the area(s) substantially free of inorganic solid particles in dry state may extend in a first direction of at least 50 μm and in a second direction of at least 50 μm, the first direction and the second direction being perpendicular to each other, which may be determined e.g. via removal of a micro-slice of water-absorbing polymer particles (via a ultramicrotome) which is then subjected to a cryo-TEM methods, known in the art or by the use of an X-ray microscopy, for example, Xradia 810 Ultra 3D X-ray Microscope commercialized by Zeiss.

The total volume of the area(s) substantially free of inorganic solid particles may be higher than the total volume of the area(s) with inorganic solid particles.

The total volume of the area(s) substantially free of inorganic solid particles is of at least 20% higher than the total volume of the area(s) with inorganic solid particles, preferably of at least 30% higher, more preferably, of at least 40% higher, even more preferably of at least 50% higher than the volume of the area(s) with inorganic solid particles.

The inorganic solid particles such as clay platelets may have a tendency to form aggregates and to hinder negatively the diffusion of fluid. Therefore, having a higher volume of area(s) substantially free of inorganic solid particles may improve the diffusion of fluid through the crosslinked polymer network. Moreover, less inorganic solid particles are needed to form the water-absorbing polymer particles of the invention, thus it may be cost-efficient.

The water-absorbing polymer particles may comprise at least two distinct areas substantially free of inorganic solid particles. By using the term “distinct”, it means that the minimum shortest distance between areas may be at least 3 μm or 10 μm or 30 μm or 50 μm or 100 μm.

The two distinct areas substantially free of inorganic solid particles may be adjacent to each other or not. The two distinct areas substantially free of inorganic solid particles may be separated from each other at a distance of at least 3 μm measured e.g. by the use of an X-ray microscopy or via removal of a micro-slice of the water-absorbing polymer particles (via a ultramicrotome) which is then subjected to a cryo-TEM methods, known in the art.

The water-absorbing polymer particles may also comprise at least two distinct areas with inorganic solid particles. The two distinct areas with inorganic solid particles may be adjacent to each other within the water-absorbing polymer particles or not.

Additionally, the water-absorbing polymer particles may comprise at least two distinct areas substantially free of inorganic solid particles and at least two distinct areas with inorganic solid particles.

The area(s) with inorganic solid particles may have a surface(s). The area(s) substantially free of inorganic solid particles may also have a surface(s).

The surface(s) of the area(s) with inorganic solid particles is (are) in contact with the surface(s) of the area(s) substantially free of inorganic solid particles.

The surface(s) of the area(s) exposed to the surrounding environment (i.e. liquid accessible surface area) may be higher for the area(s) with clay platelets compared to the area(s) substantially free of clay platelets.

Surface Crosslinking

The water-absorbing polymer particles can be post-crosslinked (i.e. surface crosslinked). Preferably, the water-absorbing polymer particles are surface crosslinked. Post-crosslinkers may include compounds comprising two or more groups capable of forming covalent bonds with the carboxylate groups of the polymers. Useful compounds include for example alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di- or polyglycidyl compounds as described in EP-A 083 022, EP-A 543 303 and EP-A 937 736, polyhydric alcohols as described in DE-C 33 14 019. Useful post-crosslinkers are further said to include by DE-A 40 20 780 cyclic carbonates, by DE-A 198 07 502 2-oxazolidone and its derivatives, such as N-(2-hydroxyethyl)-2-oxazolidone, by DE-A 198 07 992 bis- and poly-2-oxazolidones, by DE-A 198 54 573 2-oxotetrahydro-1,3-oxazine and its derivatives, by DE-A 198 54 574 N-acyl-2-oxazolidones, by DE-A 102 04 937 cyclic ureas, by DE-A 103 34 584 bicyclic amide acetals, by EP-A 1 199 327 oxetanes and cyclic ureas and by WO 03/031482 morpholine-2,3-dione and its derivatives.

Post-crosslinking is typically carried out by spraying a solution of the post-crosslinker onto the base polymer or onto the dry water-absorbing polymer particles. Spraying is followed by thermal drying, and the post-crosslinking reaction can take place not only before but also during or after drying. Preferred post-crosslinkers are amide acetals, carbamic esters, polyhydric alcohols like diols or polyols, cyclic carbonates or bisoxazolines.

At least one post-crosslinker may be used in an amount of about 1.50 wt. % or less, preferably not more than 0.50% by weight, more preferably not more than 0.30% by weight and most preferably in the range from 0.001% and 0.15% by weight based on the dry weight of the water-absorbing polymer particles.

The aqueous post-crosslinking solution, as well as the at least one post-crosslinker, can further comprise a co-solvent. Co-solvents which are technically highly useful are C₁-C₆-alcohols, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol or 2-methyl-1-propanol, C₂-C₅-diols, such as ethylene glycol, 1,2-propylene glycol, 1,3-propanediol or 1,4-butanediol, ketones, such as acetone, or carboxylic esters, such as ethyl acetate.

The total amount of post-crosslinking solution based on the base polymer may be in the range from 0.3% to 15% by weight and preferably in the range from 2% to 6% by weight.

Surface Treatment

The water-absorbing polymer particles may be coated with a surface treatment.

The coating may be done before, during or after post-crosslinking.

Such coating with one or more coating agent(s) makes it possible to achieve additional effects, such as a reduced tendency to cake, improved processing properties or a further enhanced permeability.

The surface treatment may comprise water soluble polyvalent metal salts, water-insoluble metal phosphates and inorganic particles, for example silica, clay, or mica.

Preferably, water soluble polyvalent metal salts are aluminum sulfate, aluminum nitrate, aluminum chloride, potassium aluminum sulfate, sodium aluminum sulfate, magnesium sulfate, magnesium citrate, magnesium lactate, zirconium sulfate, zirconium lactate, iron lactate, iron citrate, calcium acetate, calcium propionate, calcium citrate, calcium lactate, strontium lactate, zinc lactate, zinc sulfate, zinc citrate, aluminum lactate, aluminum acetate, aluminum formiate, calcium formiate, strontium formiate, strontium acetate. They may be used as surface treatment for the precursor water-absorbing polymer particles in order to impart a high passive fluid transport (UPM) by homogeneously coating the surface of the water-absorbing polymer particles.

Suitable water-insoluble metal phosphates may be selected from the group of pyrophosphates, hydrogen phosphates and phosphates of calcium, of magnesium, of strontium, of barium, of zinc, of iron, of aluminum, of titanium, of zirconium, of hafnium, of tin, of cerium, of scandium, of yttrium or of lanthanum, and also mixtures thereof.

Suitable inorganic particles may be applied as powders or aqueous dispersions. Inorganic particles may be selected from the group of silica, fumed silica, colloidal dispersed silica, titaniumdioxide, aluminum- and magnesiumoxide, zinc oxide, clay. Silica may be hydrophilic or hydrophobic. For example, silica is known in the art to improve the absorption speed of the precursor water-absorbing polymer particles.

The surface treatment may also be selected from the group of film-forming polymers and/or elastic polymers and/or elastic film-forming polymers. Such surface treatment may be applied in order to form a complete coating on the water-absorbing polymer particles. The term ‘film-forming’ means that the respective polymer can readily be made into a film, i.e. layer or coating, e.g. a homogeneous coating on the particle, upon evaporation of the solvent in which it is dissolved or dispersed. The polymer may for example be thermoplastic or crosslinked. Suitable film-forming polymers may exhibit elastic physical properties. The elastic and elastic film-forming agents/polymers suitable as coating agents herein are disclosed in U.S. Pat. No. 5,731,365 and in EP 0703265, and also in WO 2006/082242 and WO 2006/097389.

Method for Making Water-Absorbing Polymer Particles The First Aqueous Polymerization Solution

The first aqueous polymerization solution comprises crosslinkers, polymerizable monomers and/or oligomers and inorganic solid particles.

Preferably, the precursor water-absorbing polymer particles are obtainable by polymerization of a first aqueous polymerization solution comprising:

-   i) at least one polymerizable monomer and/or oligomer such as one     ethylenically unsaturated acid-functional monomer, -   ii) at least one crosslinker, -   iii) if appropriate one or more ethylenically and/or allylically     unsaturated monomers copolymerizable with i) and -   iv) if appropriate one or more water-soluble polymers onto which the     monomers i), ii) and if appropriate iii) can be at least partially     grafted, -   v) at least one polymerization initiator system,     wherein the base polymer obtained thereby is dried and—if     appropriate—is subsequently treated with -   vi) at least one post-crosslinker to be post-crosslinked (i.e.     surface crosslinked).

Useful polymerizable monomers and/or oligomers i) include for example ethylenically unsaturated carboxylic acids and their salts, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, tricarboxy ethylene, itaconic acid, ethylenically unsaturated phosphonic acid and ethylenically unsaturated sulfonic acid or their salts, or derivatives thereof, such as acrylamide with 2-acrylamido-2-methylpropane sulfonic acid, methacrylamide, acrylic esters and methacrylic esters.

Preferably, the polymerizable monomers and/or oligomers are selected from the group consisting of ethylenically unsaturated carboxylic acids such as methacrylic acid or its salts, or acrylic acid or its salts, ethylenically unsaturated phosphonic acids or their salts, ethylenically unsaturated sulfonic acids or their salts, or mixtures thereof.

Acrylic acid or its salts and methacrylic acid or its salts are particularly preferred monomers. Acrylic acid or its salts is most preferable.

The precursor water-absorbing polymer particles are crosslinked, i.e., the polymerization is carried out in the presence of compounds having two or more polymerizable groups which can be free-radically copolymerized into the polymer network. Useful crosslinkers ii) may include for example ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane as described in EP-A 530 438, di- and triacrylates as described in EP-A 547 847, EP-A 559 476, EP-A 632 068, WO 93/21237, WO 03/104299, WO 03/104300, WO 03/104301 and in the DE-A 103 31 450, mixed acrylates which, as well as acrylate groups, comprise further ethylenically unsaturated groups, as described in DE-A 103 31 456 and DE-A 103 55 401, or crosslinker mixtures as described for example in DE-A 195 43 368, DE-A 196 46 484, WO 90/15830 and WO 02/32962.

Preferably, the crosslinkers ii) are diacrylated, dimethacrylated, triacrylated or trimethacrylated multiply ethoxylated and/or propoxylated glycerols. Di- and/or triacrylates of 3- to 10-tuply ethoxylated glycerol are particularly advantageous. More preferably, the crosslinkers ii) are di- or triacrylates of 1- to 5-tuply ethoxylated and/or propoxylated glycerol.

Preferably, the crosslinkers comprise acrylate or acrylamide groups.

The crosslinkers of the first aqueous polymerization solution may be named “first type of crosslinkers”. Preferably, the first aqueous polymerization solution comprises a first type of crosslinkers as described above.

Examples of ethylenically unsaturated monomers iii) which are copolymerizable with the monomers i) may be acrylamide, methacrylamide, crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate and dimethylaminoneopentyl methacrylate.

Useful water-soluble polymers iv) may include polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, polyglycols, polyacrylic acids, polyvinylamine or polyallylamine, partially hydrolysed polyvinylformamide or polyvinylacetamide, preferably polyvinyl alcohol and starch.

The preparation of a suitable base polymer and also further useful hydrophilic ethylenically unsaturated monomers i) are described in DE-A 199 41 423, EP-A 686 650, WO 01/45758 and WO 03/14300.

Neutralizing agents can be used, such as alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and also mixtures thereof. Neutralizing agents may be ammonia, or amines derivatives, such as ethanolamine, diethanolamine, triethanolamine or dimethylaminoethanolamine. Sodium and potassium can be used as alkali metal salts. Preferably, neutralizing agents are sodium hydroxide, sodium carbonate or sodium bicarbonate and also mixtures thereof. Typically, neutralization is achieved by admixing the neutralizing agent as an aqueous solution or as an aqueous dispersion or else as a molten or as a solid material. The acid groups of the base polymers obtained are typically 0-100 mol %, preferably 25-100 mol %, more preferably 65-90 mol % and most preferably 68-80 mol % neutralized.

A polymerization initiator system v) is used in order to initiate the polymerization.

This polymerization initiator system may be added in solid or liquid form, for example as a solution or dispersion in a liquid such as an aqueous liquid, e.g. water.

This polymerization initiator system may comprise more than one type of compound to initiate the polymerization, or it may comprise a single type of compound.

The polymerization initiator system may include an activator, such as an activator compound or for example heat or radiation, including light radiation. Alternatively, no activation may be needed.

The polymerization initiator system can be appropriately selected from conventional (e.g. radical) polymerization initiators (and optional catalysts). Materials which display good water dispersibility/solubility are preferred. The initiator compound(s) of the system include for example peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds and redox initiators. Useful organic peroxides are for example acetylacetone peroxide, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butyl perpivalate, tert-butyl perneohexanoate, tert-butyl perisobutyrate, tert-butyl per-2-ethylhexanoate, tert-butyl perisononanoate, tert-butyl permaleate, tert-butyl perbenzoate, di(2-ethylhexyl) peroxydicarbonate, dicyclohexyl peroxydicarbonate, di(4-tert-butylcyclohexyl) peroxydicarbonate, dimyristyl peroxydicarbonate, diacetyl peroxydicarbonate, allyl peresters, cumyl peroxyneodecanoate, tert-butyl per-3,5,5-tri-methylhexanoate, acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl peroxide and tert-amyl perneodecanoate. Preferred azo compounds include 2,2′-azobisisobutyronitrile, 2,2′-azobis(2,4-dimethylvaleronitrile) and 2,2′-azobis(4-methoxy-2,4-dimethyl-valeronitrile), especially water-soluble azo initiators, examples being 2,2′-azobis-{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride, 2,2′-azobis-(2-amidinopropane)dihydrochloride,2,2′-azobis[2-(2-imidazolin-2yl)propane] dihydrochloride and 2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride. Very particular preference is given to 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride and 2,2′-azobis [2-(5-methyl-2-imidazolin-2yl)propane] dihydrochloride.

More particularly, the polymerization initiator system v) can also be persulfates such as sodium peroxodisulfate, potassium peroxodisulfate and ammonium peroxodisulfate; hydroperoxides such as t-butyl hydroperoxide and cumene hydroperoxide; and azo compounds such as 2,2′-azobis-2-amidinopropane hydrochloride, e.g. such as VA-044, V-50 and V-501 (all manufactured by Wako Pure Chemical Industries Ltd.), and mixtures of Fe²⁺; and hydrogen peroxide, or hydrogen peroxide and ascorbic acid. A mixture of two or more polymerization initiators may be used, for example one of the class of azo-compounds and one of the class of peroxo or peroxide compounds. This is believed to ensure fast polymerization. As described in US2008/242817, the use of azo compound initiator or redox initiators is advantageous for directing the rate of polymerization.

The polymerization initiator system may be introduced at a level of at least 0.001% by weight of the polymerizable monomers and/or oligomers, preferably at least 0.01%, more preferably at least 0.02%, up to 0.1%, preferably up to 0.05% by weight of the polymerizable monomers and/or oligomers.

The polymerization rate can be controlled through the identity and amount of the polymerization initiator compound used and the temperature used.

A polymerization catalyst may also be present, such as for example TMEDA (N,N,N′,N′ tetramethylethylenediamine). The polymerization of the polymerizable monomers may be highly exothermic, and hence, the polymerization liquid may be cooled during polymerization.

The precursor water-absorbing polymer particles can be post-crosslinked. Useful post-crosslinkers to post-crosslink the precursor water-absorbing polymer particles include the compounds described above for the surface-crosslinked water-absorbing polymer particles.

The weight ratio of inorganic solid particles based on the dry solid content of the first aqueous polymerization solution is from 0 to 0.5%

Preferably, the weight ratio of inorganic solid particles based on the dry solid content of the first aqueous polymerization solution is from 0 to 0.25%.

The first aqueous polymerization solution may comprise not more than 25% of the amount of inorganic solid particles comprised in the second aqueous solution, preferably not more than 10% of the amount of inorganic solid particles comprised in the second aqueous solution.

Alternatively, the first aqueous polymerization solution may comprise no inorganic solid particles, i.e. free of inorganic solid particles.

The inorganic solid particles from the first aqueous polymerization solution may be clay platelets. Preferably, the clay platelets are surface and/or edge modified.

The Second Aqueous Solution

The second aqueous solution comprises inorganic solid particles, crosslinkers, and either polymerizable monomers and/or oligomers, or crosslinkable polymers.

The crosslinkers of the second aqueous solution may be a first type of crosslinkers and/or a second type of crosslinkers.

Preferably, the second aqueous solution comprises inorganic solid particles, a second type of crosslinkers and crosslinkable polymers.

The second aqueous solution may comprise water. The solution may comprise further solvents in addition to water, such as organic solvent.

The concentration of inorganic solid particles such as clay platelets in the second aqueous solution may be less than 20% by weight of the dispersion, or less than 10% by weight of the dispersion, or less than 5% by weight of the dispersion or less than 1% by weight of the dispersion.

The concentration of inorganic solid particles such as clay platelets in the second aqueous solution may be at least 0.5% by weight of the dispersion, or at least 1% by weight of the dispersion, or at least 3% by weight of the dispersion.

The weight ratio of inorganic solid particles based on the dry solid content of the second aqueous solution is more than 0.5%, preferably more than 1%, even more preferably more than 5%.

Inorganic Solid Particles: Clay and Clay Platelets

As mentioned before, inorganic solids particles may be clay particles. The clay particles may be in the form of platelets. The inorganic solid particles from the second aqueous solution may be clay platelets. Preferably, the clay platelets are surface and/or edge modified.

The present invention applies clay that can be dispersed as platelets in an aqueous solution such as an acidic aqueous liquid.

The clay platelets in the second aqueous solution are preferably homogeneously dispersed, e.g. there is no significant aggregation or flocculation of the clay platelets.

The surface and/or edge-modified clay platelets in the second aqueous solution may have an aspect ratio of less than 300, preferably less than 200, more preferably less than 100. The aspect ratio of the surface and/or edge-modified clay platelets in the second aqueous solution is generally more than 5, preferably more than 10.

The aspect ratio of clay platelet is the ratio of the largest dimension and the lowest dimension, orthogonal to it, of the clay platelet.

When the clay platelets have a large size dimension, it may be beneficial to break the larger size clay platelets by using an ultrasonic treatment before assessing their weight average largest particle size dimension as described above.

In the second aqueous solution, the clay platelets may be present as individual platelets or may be present as small aggregates of, for example, 2 to 5 clay platelets which may be determined via removal of a micro-slice of the water-absorbing polymer particles (via a ultramicrotome) which is then subjected to a cryo-TEM methods, known in the art or by the use of the dynamic light scattering test method.

The clay platelets may be purified before surface-modification and/or edge-modification, e.g. to remove metals etc., by methods known in the art. For example, the clay to be modified may be a di-octahedral or tri-octahedral clay.

The clay platelets may have modified basal surface and/or modified edges. The surface and/or edge modification of the clay platelets may be done prior to adding the polymerizable monomers and/or oligomers or crosslinkable polymers, or simultaneously with adding the polymerizable monomers and/or oligomers or crosslinkable polymers. To obtain the surface and/or edge-modified clay platelets, the clay platelets may be dispersed in a solution that comprises the surface modification compound(s) and/or the edge modification compound(s), and/or the clay platelets may be dispersed in a solution, and the modification compound(s) may then be added to the dispersion, optionally also as solution or dispersion.

The ratio of clay platelets to the surface modification compound(s) and/or the edge modification compound(s) may be within the range of 1:1 to 100:1 (by weight, based on the weight of dry clay platelets and dry edge and/or surface modification compound(s)).

In the following, the surface modification compound(s) and/or the edge modification compound(s) are described as they are before addition to the clay platelets.

Edge Modification Compound(s)

When modifying the edges of the clay platelets, the exchangeable cations of the clay platelet edges may be replaced by the edge modification compound(s). Then, typically, the point of zero charge of the clay platelet edges is either shifted to a lower pH value, or the edge charge is made pH-independently neutral or pH-independently negative.

In addition, or alternatively, the edge modification compound may be a compound, which hinders and reduces aggregation of clay platelets.

The edge modification compound(s) may consist of one or more phosphorylation compounds. The phosphorylation compound(s) may be selected from the group consisting of: phosphate salts and/or derivatives thereof and/or acids forms thereof condensed phosphate salts, and/or derivatives thereof and/or acids forms thereof; phosponic acid, derivatives thereof and salts thereof and combinations thereof. For example, sodium pyrophosphate decahydrate may be suitably used. Organo-phosphor derivatives may also be useful.

The edge modification compound(s) may consist of one or more silanization compounds (also referred to as: silane compound).

The silanization compound may be an organo silane compound, e.g. of the formula: SiR^(I)R^(II)R^(III)R^(IV), whereby the moieties R^(I), R^(II), R^(III), R^(IV) are each selected from the group consisting of the subgroups: a) Alkyl, Aryl, N-Alkyls, Alkenes, alkenyls; and b) Alkoxy, hydrogen, toluenesulfonyl, sulfonyl containing moieties, chloride, halide; and c) hydroxy, carboxy-containing moieties, epoxy-containing moieties, provided that at least one moieties are selected from the subgroup b) or subgroup c) and that not more than three moieties are selected from said subgroup a).

Preferably, the silanization compound may be an organo silane compound, e.g. of the formula: SiR^(I)R^(II)R^(III)R^(IV), whereby the moieties R^(I), R^(II), R^(III), R^(IV) are each selected from the group consisting of the subgroups: a) Alkyl, Aryl, N-Alkyls, Alkenes, alkenyls; and b) Alkoxy, hydrogen, toluenesulfonyl, sulfonyl containing moieties, chloride, halide; and c) hydroxy, carboxy-containing moieties, epoxy-containing moieties, provided that at least from one to three moieties are selected from the subgroup a) and that at least one moieties are selected from the subgroup b) or subgroup c).

It may be beneficial that at least one of said moieties R^(I), R^(II), R^(III), R^(IV) is a moiety that is suitable to bond to the polymerizable monomer or polymerizable oligomer. For example, at least one of said moieties is an unsaturated moiety, such as vinyl. Preferably, the edge modification compound(s) is a silanization compound such as 7-Octenyldimethylmethoxysilane.

The edge modification compound(s) may consist of one or more fluorination compounds. Preferably, the edge modification compound(s) include fluoride salt. Preferably, the counterion M is a mono-valent counterion, such as sodium or ammonium.

The edge modification compound(s) may be a compound that sterically hinders from the aggregation of said platelet edges in order to reduce the risk of aggregation of the clay platelets in the solution, in addition to modifying the charge of the edges of the clay.

The edge modification compound(s) may have at least one moiety of at least 10 angstrom (A) or of at least 15 angstrom, or of at least 20 angstrom. Preferably the edge modification compound(s) have at least a moiety with a carbon chain of at least 6 carbon atoms, or at least 9 carbon atoms or at least 12 carbon atoms.

Other compounds to modify the edges of the clay platelets include epoxides. For example polyether clay platelets can be formed.

Preferably, the edge modification compound(s) consist(s) of one or more phosphorylation compounds, silanization compounds or fluorination compounds or combination thereof.

The edge-modification compound, in particular those described above as phosphorization, silanization or fluorination compounds, may have a further moiety or moieties that can ionically or covalently bind to the monomer or oligomer, or to the polymer formed; for example, the edge modification compound may have one or more unsaturated moieties (e.g. with C═C group), and/or one or more moieties that can form an ester or amide bond with the carboxyl group of the monomer, oligomer or polymer thereof, such as an oligo-ether or polyether moiety. Then, the edge modification compound not only binds to the edge of the clay platelet, but the compound(s) can also ionically or covalently bind to the polymers.

The clay platelets may not only be edge-modified to ensure homogeneous dispersion but the edge modification may further serve to strongly bind the clay platelets to the polymerizable monomers and/or oligomers or crosslinkable polymers, e.g. covalently or ionically.

Surface Modification Compound(s)

The surface modification compound(s) may be a compound that has a cationic moiety (and/or: cationic at the pH of the liquid herein and reaction herein), that can bind to the negatively charged basal surface of the clay platelet. The surface modified clay may have surface(s) that are neutral (at the pH of the carrier liquid).

The surface modification compound(s) may comprise an alkylated nitrogen moiety, or alkoxylated nitrogen moiety, including for example linear, branched or cyclic amino-, ammonium-compounds. A majority of the moieties may be cationic at the pH of the reaction liquid/reaction.

The surface modification compound(s) may have one or more moieties selected from amines or imines, including derivatives thereof, such as diamines or diimines and/or ethylene or poly- or oligo-ethylene derivatives thereof, including hexamethylene diamine and derivatives thereof, ethylendiamine and derivatives thereof, oligo-alkyleneimine and derivatives thereof, such as linear or branched polyethyleneimine, olig-etheramines and derivatives thereof, linear or branched amides, or mixtures thereof.

The surface modification compound(s) may have an acryl amide moiety. The surface modification compound(s) may have a urethane moiety (bond by hydrogen bonding to the negative basal surface) or further modifications thereof. Preferably, the surface modification compound(s) may have a cationically modified urethane moiety.

Especially preferred are moieties selected from linear or branched polyethyleneimine, hexamethylene diamine or ethylendiamine, or derivatives of any of these, or mixtures thereof.

The surface modification compound(s) may also be a cationically modified oligo- or poly-saccharides, or derivative thereof.

In addition, the surface modification compound(s) may have one or more further moiety that is or are hydrophilic. This can aid dispersion of the surface-modified clay in the solution and/or can further enhance the hydrophilicity, and hence affinity for hydrophilic fluids (e.g. urine, blood, saline water), of the water-absorbing polymer particles. This may for example be anionic moiety, or —OH. Preferably, the surface modification compound(s) has at least one moiety that is an alkoxylated moiety, carboxylated moiety, or sulfonated moiety, or sulfated moiety, to further improve hydrophilicity.

The surface modification compound(s) may be such that, when chemically bound (for example electrostatic bond) to the clay platelet surfaces, they introduce a sterically hindering moiety (s), which hinders and hence reduces aggregation of clay platelets. Hence, the surface modification compound(s) may have a moiety that is sterically hindering aggregation. Preferably, the surface modification compound(s) has one or more moieties that can provide sterical hindrance, having at least 6 carbon atoms, and/or a length of at least 10 angstrom, or at least 15 angstrom. Preferably, the surface modification compound(s) has an oligomer chain moiety.

For example, the surface modification compound(s) may have oligo-alkyleneoxide (AO) moiety, such as a oligo-ethyleneoxide (EO) moiety, with an average number of AO (e.g. EO)-repeating units of at least 2 or at least 5 or at least 10, and up to 100, or up to 60 or up to 40. Preferably, the surface modification compound(s) has at least a moiety that is an oligo-ethoxylate with a number of 2 to 40 repeating units.

The surface modification compound(s), in particular those with a cationic group as described above, may have a further moiety or moieties that can ionically or covalently bind to the monomer or oligomer, or to the polymer formed thereby; for example, the surface modification compound may have one or more unsaturated moieties (e.g. with C═C group), and/or one or more moieties that can form an ester or amide bond with the carboxyl group of the monomer, oligomer or polymer thereof, such as an oligo-ether or polyether moiety. Then, the surface modification compound not only binds to the surface of the clay platelet, but the compound(s) can also ionically or covalently bind to the polymers. Thus, the clay platelets are not only surface-modified to ensure homogeneous dispersion but the surface modification further serves to strongly bind to the polymers, e.g. covalently or ionically. The surface modification compound described herein above, e.g. with a cationic group, may for example comprise a polymerizable moiety, such as an alkylene, e.g. ethylene; and/or the unsaturated moiety may for example be an ester of acrylic acid, and/or an alkylated derivatives of acrylic acid, such as methacrylic acid.

It may be useful to apply during the surface and/or edge modification step and/or after the surface and/or edge modification step, an ultrasonic treatment step, and/or mixing step; preferred is the application of (e.g. high) shear mixing. For example, a Y-Tron mixer can be used. The exfoliation of the clay platelet may also be affected by use of high-shear mixers, (such as CB Loedige mixers, Schugi mixers, Littleford mixers, Drais mixers). The tip speed of any such mixers may for example be from at least 20 ms⁻¹, or at least 30 ms⁻¹ to for example 45 or 40 or 35 ms⁻¹.

The surface and/or edge modification of the clay platelets may be done in any liquid. It may for example be done in water. Alternatively, the surface and/or edge modification may be done in the absence of water, e.g. preferably in an anhydrous liquid, e.g. anhydrous liquid with a dielectric constant larger than 40 preferentially more than 50, for example propylene carbonate or ethylene carbonate. Preferred may be that the liquid phase comprises at least 80% by weight of water, preferably at least 90% by weight or even 100% by weight of water.

Preferably, the surface and/or edge modification compound(s) modify the clay platelets prior to mixing with the polymerizable monomers and/or oligomers or crosslinkable polymers. It may be preferred to modify the clay platelet's surfaces and/or edge, and then to wash the resulting modified clay platelet, and/or filtrate and or/submit to dialysis the modified clay platelets, prior to mixing with the polymerizable monomers and/or oligomers or crosslinkable polymers.

Polymerizable Monomers and/or Oligomers or Crosslinkable Polymers

The second aqueous solution may comprise polymerizable monomers and/or oligomers which contain a multiplicity of functional groups such as charges groups (anionic, cationic) for example carboxylic acid groups or their salts, preferably sodium salts.

Preferably, the polymerizable monomers and/or oligomers comprise polymerizable monomers and/or oligomers of acrylic acids or their salts or acrylates or derivatives thereof.

Polymerizable monomers and/or oligomers may include for example ethylenically unsaturated carboxylic acids or their salts, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, tricarboxy ethylene, itaconic acid, ethylenically unsaturated phosphonic acid or their salts, ethylenically unsaturated sulfonic acid or their salts, or derivatives thereof, such as acrylamide with 2-acrylamido-2-methylpropane sulfonic acid, methacrylamide, acrylic esters and methacrylic esters.

Preferably, the polymerizable monomers and/or oligomers are selected from the group consisting of ethylenically unsaturated carboxylic acids such as methacrylic acid or its salts, or acrylic acid or its salts, ethylenically unsaturated phosphonic acids or their salts, ethylenically unsaturated sulfonic acids or their salts, or mixtures thereof.

Acrylic acid or its salts and methacrylic acid or its salts are particularly preferred polymerizable monomers and/or oligomers. Acrylic acid or its salts is most preferable.

The preparation of useful polymerizable monomers and/or oligomers are described in DE-A 199 41 423, EP-A 686 650, WO 01/45758 and WO 03/14300.

The polymerizable monomers and/or oligomers may be used in the solution at a level of at least 1% by weight to 90% by weight, preferably from 10% by weight to 60% by weight.

In addition to polymerizable monomers and/or oligomers, the second aqueous solution may also comprise one or more polymerizable ethylenically and/or allylically unsaturated monomers copolymerizable with the polymerizable monomers and/or oligomers, e.g. polymerizable ethylenically unsaturated acid-functional monomers or their derivatives. Examples of copolymerizable ethylenically unsaturated monomers may be acrylamide, methacrylamide, crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate and dimethylaminoneopentyl methacrylate.

Alternatively, the second aqueous solution may comprise crosslinkable polymers. Preferably, the second aqueous solution comprises crosslinkable polymers. The crosslinkable polymers may include polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, polyglycols, polyacrylic acids, polyvinylamine or polyallylamine, partially hydrolysed polyvinylformamide or polyvinylacetamide. Preferably, the crosslinkable polymers are polyacrylic acids or their salts or polyacrylates or derivatives thereof.

The crosslinkable polymers may have a weight average molecular weight determined by gel permeability chromatography of more than 8,000 g/mol, preferably within the range of 10,000 g/mol to 1,000,000 g/mol, more preferably within the range of 50,000 to about 750,000 g/mol and even more preferably within the range of 90,000 to 700,000 g/mol.

In addition to polymerizable monomers and/or oligomers, the second aqueous solution may also comprise neutralizing agents. Neutralizing agents may be used, such as alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and also mixtures thereof. Neutralizing agents may be ammonia, or amines derivatives, such as ethanolamine, diethanolamine, triethanolamine or dimethylaminoethanolamine. Sodium and potassium can be used as alkali metal salts. Preferably, neutralizing agents are sodium hydroxide, sodium carbonate or sodium bicarbonate and also mixtures thereof. Typically, neutralization is achieved by admixing the neutralizing agent as an aqueous solution or as an aqueous dispersion or else as a molten or as a solid material. The acid groups of the polymerizable monomers and/or oligomers are typically 0-100 mol %, preferably 25-100 mol %, more preferably 65-90 mol % and most preferably 68-80 mol % neutralized.

The second aqueous solution can further comprise a co-solvent. Co-solvents which are technically highly useful are C₁-C₆-alcohols, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol or 2-methyl-1-propanol, C₂-C₅-diols, such as ethylene glycol, 1,2-propylene glycol, 1,3-propanediol or 1,4-butanediol, ketones, such as acetone, or carboxylic esters, such as ethyl acetate.

The second aqueous solution can further comprise additives such as polyethylene glycol, polypropylene glycol, mixed polyalkoxylates, polyalkoxylates based on polyols such as glycerine, trimethylolpropane or butanediol, surfactants with a HLB of more than 10 such as alkyl polyglucosides or ethoxylated sugar esters such as polysorbates. The additives may reduce the hardness or the brittleness of the water-absorbing polymer particles obtained by the method described herein.

First Type of Crosslinkers

The second aqueous solution may comprise a first type of crosslinkers. The first type of crosslinkers may include for example ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane as described in EP-A 530 438, di- and triacrylates as described in EP-A 547 847, EP-A 559 476, EP-A 632 068, WO 93/21237, WO 03/104299, WO 03/104300, WO 03/104301 and in the DE-A 103 31 450, mixed acrylates which, as well as acrylate groups, comprise further ethylenically unsaturated groups, as described in DE-A 103 31 456 and DE-A 103 55 401, or crosslinker mixtures as described for example in DE-A 195 43 368, DE-A 196 46 484, WO 90/15830 and WO 02/32962.

Preferably, the first type of crosslinkers comprises acrylate or acrylamide groups.

When the second aqueous solution comprises a first type of crosslinkers, the second aqueous solution also comprises polymerizable monomers and/or oligomers.

Preferably, the second aqueous solution comprises a first type of crosslinkers with polymerizable monomers and/or oligomers.

Preferably, the second aqueous solution comprises acrylate or acrylamide groups with polymerizable monomers and/or oligomers

Preferably, the first type of crosslinkers is diacrylated, dimethacrylated, triacrylated or trimethacrylated multiply ethoxylated and/or propoxylated glycerols. Di- and/or triacrylates of 3- to 10-tuply ethoxylated glycerol are particularly advantageous. More preferably, the first type of the crosslinkers is di- or triacrylates of 1- to 5-tuply ethoxylated and/or propoxylated glycerol.

When the second aqueous solution comprises a first type of crosslinkers, the second aqueous solution may also comprise a polymerization initiator system in order to initiate the polymerization as described above for the first aqueous polymerization solution.

Second Type of Crosslinkers

Alternatively or in addition to the first type of crosslinkers, the second aqueous solution may comprise a second type of crosslinkers. The second type of crosslinkers may include compounds comprising two or more groups capable of forming covalent bonds with the carboxylate groups of the polymers. Useful compounds include for example alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di- or polyglycidyl compounds as described in EP-A 083 022, EP-A 543 303 and EP-A 937 736, polyhydric alcohols as described in DE-C 33 14 019. A second type of crosslinkers may include the compounds from DE-A 40 20 780 cyclic carbonates, from DE-A 198 07 502 2-oxazolidone and its derivatives, such as N-(2-hydroxyethyl)-2-oxazolidone, from DE-A 198 07 992 bis- and poly-2-oxazolidones, from DE-A 198 54 573 2-oxotetrahydro-1,3-oxazine and its derivatives, from DE-A 198 54 574 N-acyl-2-oxazolidones, from DE-A 102 04 937 cyclic ureas, from DE-A 103 34 584 bicyclic amide acetals, from EP-A 1 199 327 oxetanes and cyclic ureas and from WO 03/031482 morpholine-2,3-dione and its derivatives.

Preferred second type of crosslinkers are amide acetals, carbamic esters, polyhydric alcohols like diols or polyols, cyclic carbonates, bisoxazolines, epoxides or Glycidyl Ethers such as Ethylene Glycol Diglycidyl Ether.

Preferred second type of crosslinkers are Glycidyl Ethers such as Ethylene Glycol Diglycidyl Ether.

When the second aqueous solution comprises a second type of crosslinkers, the second aqueous solution also comprises crosslinkable polymers.

Preferably, the second aqueous solution comprises a second type of crosslinkers with crosslinkable polymers.

Preferably, the second aqueous solution comprises Glycidyl Ethers such as Ethylene Glycol Diglycidyl Ether with crosslinkable polymers.

The second aqueous solution can comprise the second type of crosslinkers in a quantity within the range of 0.001 wt. % to 30 wt. %, preferably within the range of 0.01 wt. % to 15 wt. %, more preferably within the range of 0.02 wt. % to 7 wt. % based on the weight of the solution.

The second aqueous solution comprising inorganic solid particles and polymerizable monomers and/or oligomers or crosslinkable polymers may have a viscosity determined according to ASTM 1824/90 at about 20° C. within a range of 25 mPa·s to 50,000 mPa·s, preferably within a range of 25 mPa·s to 20,000 mPa·s, more preferably within a range of 25 mPa·s to 5,000 mPa·s.

The high viscosity of the second aqueous solution comprising polymerizable monomers and/or oligomers or crosslinkable polymers allows the neighboring precursor water-absorbing polymer particles, when mixed with the second aqueous solution as described above, to be fixed together at specific points of contact between the particles. At these specific points of contact, the high viscosity of the solution enables the neighboring precursor water-absorbing polymer particles to be fixed together. The high viscosity of the solution comprising polymerizable monomers and/or oligomers or crosslinkable polymers may help to form water-absorbing polymer particles wherein at least two areas substantially free of inorganic solid particles are distinct. Relative strong bonds are created between the neighboring precursor water-absorbing polymer particles resulting in the water-absorbing polymer particles.

The second aqueous solution may comprise from 0.1 to 10 wt % of inorganic solid particles such as clay platelets with modified surfaces and/or edges, from 5 to 95 weight % of water; from 5 to 95 wt. % of polymerizable monomers and/or oligomers, from 0.001 to 10 wt. % of a first type of crosslinkers, optionally a dispersing aid, and from 0.001 to 5 wt. % of polymerization initiator to start the polymerization.

Alternatively, the second aqueous solution may comprise from 0.1 to 10 wt % of inorganic solid particles such as clay platelets with modified surfaces and/or edges, from 5 to 95 weight % of water; from 5 to 95 weight % of crosslinkable polymers, from 0.001 to 10 weight % of a second type of crosslinkers and optionally a dispersing aid.

Polymerization and Mixing Steps

The first aqueous polymerization solution may be polymerizable by any type of polymerization reaction, by use of a polymerization initiator system that is activated, to initiate the polymerization. The polymerization initiator system may be activated by applying heat and/or radiation.

After the polymerization of the first aqueous polymerization solution, precursor water-absorbing polymer particles are obtained.

If desired, the polymerization step can be followed by a drying step, e.g. at temperatures of e.g. more than 50° C., more than 100° C., more than 120° C., more than 180° C. or more than 200° C. or preferably of 100° C. to 150° C.

The precursor water-absorbing polymer particles obtained after the polymerization of the first aqueous polymerization solution are mixed with the second aqueous solution comprising inorganic solid particles such as clay platelets with modified surfaces and/or edges and polymerizable monomers and/or oligomers or crosslinkable polymers.

Mixing of the precursor water-absorbing polymer particles with the second aqueous solution comprising inorganic solid particles such as clay platelets with modified surfaces and/or edges may be done with a large volume of precursor water-absorbing polymer particles and of second aqueous solution.

Alternatively, by spraying the second aqueous solution onto the precursor water-absorbing polymer particles, mixing of the precursor water-absorbing polymer particles with the second aqueous solution comprising inorganic solid particles such as clay platelets with modified surfaces and/or edges may be done at a rate of at least 1 g, or at least 2 g, or at least 4 g, or at least 6 g, or at least 8 g of solution per kg of provided precursor water-absorbing polymer particles per minute. The rate may be less than 200 g/kg/min, or less than 100 g/kg/min.

It may be desirable to agitate the precursor water-absorbing polymer particles during and/or after the second aqueous solution is applied.

Mixing can be done in equipment well known in the art, such as coaters, paddle mixers, ploughshare mixers, kneaders, fluidized bed coaters, Wurster coaters, spinning disk reactors, etc.

Mixing can be done at room temperature or at elevated temperatures (e.g. around 30° C. to 100° C., more preferably around 40° C. to 70° C.).

If the second aqueous solution comprises a first type of crosslinkers, a polymerization step will be necessary. For example, the polymerization step may be a radical polymerization step.

The mixed solution comprising precursor water-absorbing polymer particles, inorganic solid particles such as clay platelets with modified surfaces and/or edges, polymerizable monomers and/or oligomers and a first type of crosslinkers may be polymerizable by any type of polymerization reaction, by use of a polymerization initiator system that is activated, to initiate the polymerization. The polymerization initiator system may be activated by applying heat (at a temperature of 120° C. or higher than 120° C.) and/or radiation. After the polymerization, water-absorbing polymer particles of the invention are obtained.

If desired, polymerization step can be followed by a drying step, for example, at temperatures of e.g. more than 50° C., more than 100° C., more than 120° C., more than 180° C. or more than 200° C. or preferably of 100° C. to 150° C.

If the second aqueous solution comprises a second type of crosslinkers being amide acetals, carbamic esters, polyhydric alcohols like diols or polyols, cyclic carbonates or bisoxazolines or epoxides, a polymerization step may not be necessary. For example, a radical polymerization step may not be necessary.

If the second aqueous solution comprises a second type of crosslinkers, a crosslinking step follows the mixing step.

A crosslinking step may be done with the mixed solution comprising precursor water-absorbing polymer particles, inorganic solid particles such as clay platelets with modified surfaces and/or edges, crosslinkable polymers and a second type of crosslinkers.

The crosslinking step may be a heating step at a temperature of 120° C. or higher than 120° C. or a drying step. After the crosslinking step, water-absorbing polymer particles of the invention are obtained.

If desired, crosslinking step can be followed by a drying step, for example, at temperatures of e.g. more than 50° C., more than 100° C., more than 120° C., more than 180° C. or more than 200° C. or preferably of 100° C. to 150° C.

The water-absorbing polymer particles obtained by the polymerization step or the crosslinking step and the optional drying step are desirably dry water-absorbing polymer particles. The dry water-absorbing polymer particles may have a water content of less than 10% by weight, or less than 5% by weight, or less than 3% by weight.

Formula

The average closest distance between two neighboring crosslinkers (R_(XL) in cm) from the first aqueous polymerization solution or from the second aqueous solution for a specific X-load of a water-absorbing polymer particle is calculated via the formula below:

$\begin{matrix} {{Rxl} = \left( \frac{\left( {\frac{1}{{rho}_{—}{dry}} + \frac{x_{—}L}{{rho}_{—}{liq}}} \right)}{N_{A} \cdot {\sum\limits_{i}\frac{w_{—}{xl}_{i}}{{Mr}_{—}{CXL}_{i}}}} \right)^{\frac{1}{3}}} & (I) \end{matrix}$

with x_L corresponding to the amount of liquid absorbed in the water-absorbing polymer particle in g _(liq)/g _(water-absorbing polymer particle),

rho_liq corresponding to the density at room temperature of the fluid that swells the water-absorbing polymer particle (generally saline of 0.9% w NaCl) in g/cm³,

rho_dry corresponding to the true density of the dry water-absorbing polymer particle formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively, in g/cm³,

Mr_CXL corresponding to the molar mass of the crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in g/mol,

w_xl corresponding to the weight ratio of crosslinkers in dry water-absorbing polymer particle from the first aqueous polymerization solution or from the second aqueous solution, respectively,

N_(A) corresponding to the Avogadro's number in mol⁻¹.

The formula above has been obtained as follows:

The average closest distance between two neighboring crosslinkers (Rxl in cm) from the first aqueous polymerization solution or from the second aqueous solution for a specific X-load of a water-absorbing polymer particle may be calculated via the formula below:

$\begin{matrix} {{Rxl} = \left( \frac{V_{—}{gel}}{Nxl} \right)^{1\text{/}3}} & ({II}) \end{matrix}$

with V_gel corresponding to the volume of the gel at given X-load in cm³, and

N_(xl) corresponding to the number of all crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in the cubic gel volume (V_gel) and as such it is the sum of all crosslinkers from the first aqueous polymerization solution or from the second aqueous solution present, respectively, in the cubic gel volume.

When more than one crosslinker types is present in the cubic gel volume, N_(xl) corresponds to Σ_(i) Nxl_(i).

The underlying assumption is that crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, are uniformly distributed in space in a cubic gel volume (V_gel).

The number of all crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in the cubic gel volume (N_(xl)) may be calculated via the formula below:

Nxl=n_xlN_A  (III)

with N_A corresponding to the Avogadro's number in mol-1, and

n_xl corresponding to the moles of crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in mol and may be calculated via the formula below:

$\begin{matrix} {{n_{—}{xl}} = \frac{m_{—}{xl}}{{Mr}\mspace{14mu} {CXL}}} & ({IV}) \end{matrix}$

having Mr_CXL corresponding to the molar mass of the crosslinkers in g/mol, and

m_xl is the mass of crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in g and may be calculated via the formula below:

$\begin{matrix} {{w_{—}{xl}} = \frac{m_{—}{xl}}{m_{—}{dry}}} & (V) \\ {{{so}\mspace{14mu} m_{—}{xl}} = {w_{—}{xl}\mspace{14mu} m_{—}{dry}}} & ({Va}) \end{matrix}$

with w_xl corresponding to the weight ratio of crosslinkers in dry water-absorbing polymer particles from the first aqueous polymerization solution or from the second aqueous solution, respectively, and

m_dry corresponding to the mass of dry water-absorbing polymer particles formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively, in g.

Therefore, by substituting formula (IV) and (Va) into formula (III), the number of all crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in the cubic gel volume (N_(xl)) may be calculated via the formula below:

$\begin{matrix} {{Nxl} = \frac{w_{—}{xl}\mspace{14mu} m_{—}{dry}\mspace{14mu} N_{—}A}{{Mr}\mspace{14mu} {CXL}}} & ({VI}) \end{matrix}$

V_gel corresponds to the approximation of the gel volume as sum of the volume of the dry water-absorbing polymer particle (V_dry) formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively, and of the volume of the absorbed liquid (V_liq), i.e. assuming that liquid is linearly absorbed during swelling of the water-absorbing polymer particle, and is calculated via the equation below:

$\begin{matrix} {{V_{—}{gel}} = {{V_{—}{dry}} + {V_{—}{liq}}}} & ({VII}) \\ {{{With}\mspace{14mu} V_{—}{dry}} = \frac{m_{—}{dry}}{{rho}\mspace{14mu} {dry}}} & ({VIII}) \end{matrix}$

-   -   having m_dry corresponding to the mass of dry water-absorbing         polymer particle formed out of the first aqueous polymerization         solution and out of the second aqueous solution, respectively,         in g,         -   rho_dry corresponding to the true density of the dry             water-absorbing polymer particle formed out of the first             aqueous polymerization solution and out of the second             aqueous solution, respectively, in g/cm³, and

$\begin{matrix} {{V_{—}{liq}} = \frac{m_{—}{liq}}{{rho}_{—}{liq}}} & ({IX}) \end{matrix}$

-   -   having m_liq corresponding to the mass of swelling liquid in g,         and         -   rho_liq corresponding to the density at room temperature of             the fluid that swells the water-absorbing polymer particle             (generally saline of 0.9% w NaCl) in g/cm³.

The amount of liquid absorbed in the water-absorbing polymer particle formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively, (in g _(liq)/g _(dry water-absorbing polymer particle)) (x_L) is calculated via the formula below:

$\begin{matrix} {{x_{—}L} = \frac{m_{—}{liq}}{m_{—}{dry}}} & (X) \\ {{{so}\mspace{14mu} m_{—}{liq}} = {x_{—}{L \cdot m_{—}}{dry}}} & ({Xa}) \end{matrix}$

With m_dry corresponding to the mass of dry water-absorbing polymer particle formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively, in g, and

m_liq corresponding to the mass of swelling liquid in g.

The X-load of the water-absorbing polymer particles formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively, is the amount of liquid absorbed by the water-absorbing polymer particles.

Therefore, by substituting formula (Xa) into formula (IX) and into formula (VII), and substituting formula (VIII) into formula (VII) and then both formula (VII) and (VI) into formula (II), we obtain the average closest distance between two neighboring crosslinkers (Rxl) from the first aqueous polymerization solution or from the second aqueous solution for a specific X-load of a water-absorbing polymer particle calculated via the formula below:

$\begin{matrix} {{Rxl} = \left( \frac{\left( {\frac{1}{{rho}_{—}{dry}} + \frac{x_{—}L}{{rho}_{—}{liq}}} \right)}{N_{A} \cdot \frac{w_{—}{xl}}{{Mr}_{—}{CXL}}} \right)^{\frac{1}{3}}} & ({XI}) \end{matrix}$

with x_L corresponding to the amount of liquid absorbed in the water-absorbing polymer particle formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively in g _(liq)/g _(water-absorbing polymer particle),

rho_liq corresponding to the density at room temperature of the fluid that swells the water-absorbing polymer particle (generally saline of 0.9% w NaCl) in g/cm³,

rho_dry corresponding to the true density of the dry water-absorbing polymer particle formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively in g/cm³,

Mr_CXL corresponding to the molar mass of the crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in g/mol,

w_xl corresponding to the weight ratio of crosslinkers in dry water-absorbing polymer particle from the first aqueous polymerization solution or from the second aqueous solution, respectively,

N_(A) corresponding to the Avogadro's number in mol⁻¹.

When more than one crosslinker is used, the average closest distance between two neighboring crosslinkers (Rxl) from the first aqueous polymerization solution or from the second aqueous solution for a specific X-load of a water-absorbing polymer particle is calculated via the formula below:

$\begin{matrix} {{Rxl} = \left( \frac{\left( {\frac{1}{{rho}_{—}{dry}} + \frac{x_{—}L}{{rho}_{—}{liq}}} \right)}{N_{A} \cdot {\sum\limits_{i}\frac{w_{—}{xl}_{i}}{{Mr}_{—}{CXL}_{i}}}} \right)^{\frac{1}{3}}} & (I) \end{matrix}$

Depending on the X-load of the water-absorbing polymer particle, the average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution in the water-absorbing polymer particle varies.

The X-load of the water-absorbing polymer particle formed out of the first aqueous polymerization solution and out of the second aqueous solution, respectively, may be for example 0 g/g, i.e. corresponding to the dry state of the water-absorbing polymer particles, 10 g/g, 20 g/g or 30 g/g.

The average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle obtained via the formula above is at least as high as an average size of the inorganic solid particles or higher than an average size of the inorganic solid particles.

Therefore, the inorganic solid particles present an average range of size that can fit the average closest distance between two neighboring crosslinkers (R_(XL)) in the water-absorbing polymer particle at 20 g/g X-load of the water-absorbing polymer particle.

The average range of size of the inorganic solid particles may be determined by measuring the diameter distribution of the inorganic solid particles by static Dynamic Light Scattering method.

The dynamic light scattering test method is described in the article: Karpovich, A et al, “Determination of dimensions of exfoliating materials in aqueous suspensions”, MethodsX, 2016, 3, 19-24. NMR relaxometry test method may also be used and is described in the same article above.

In particular, for inorganic solid particles with a lateral size over 100 nm, the most appropriate test method may be the static and dynamic light scattering method in aqueous suspension as described in the article of Allen, T; Particle Size Measurement, Volume 1: Powder sampling and particle size measurement, 1997.

For inorganic solid particles with a lateral size below 100 nm, the most appropriate test method may be the Atomic Force Microscopy (AFM) method which is described in the article of Cadene et al, Study of individual Na-montmorillonite particle size, morphology, and apparante charge, Journal of Colloid and Interface Science, 2005, 285, 719-730 and in Balnois et al, Langmuir 2003, 19, 6633.

“D10” is the particle size of the inorganic solid particles where 10% by weight of the particles are finer than this size according to the dynamic light scattering method known in the art.

“D50” is the particle size of the inorganic solid particles where 50% by weight of the particles are finer than this size according to the dynamic light scattering method described above.

“D90” is the particle size of the inorganic solid particles where 90% by weight of the particles are finer than this size according to the dynamic light scattering method described above.

“D84” is the particle size of the inorganic solid particles where 84% by weight of the particles are finer than this size according to the dynamic light scattering method described above.

Preferably, the average size of the inorganic solid particles is approximately equal to the value of the particle size D50 according to the dynamic light scattering method described above.

The average value of the particle size D50 may be 14 nm. So, the average size of the inorganic solid particles may be approximately equal to 14 nm according to the dynamic light scattering method described above.

Preferably, the inorganic solid particles of the invention have an average range of size that encloses the average size of the inorganic solid particles.

The average range of size of the inorganic solid particles may be from 3 to 100 nm according to the dynamic light scattering method described above.

Preferably, the average range of size of the inorganic particles may be from 3 to 80 nm, more preferably from 3 to 70 nm and even more preferably from 3 to 50 nm according to the dynamic light scattering method described above.

Preferably, the average range of size of the inorganic particles is from 10 to 20 nm according to the dynamic light scattering method described above.

Alternatively, the average range of size of the inorganic solid particles may correspond to the range of values between D10 and D90.

Alternatively, the average range of size of the inorganic solid particles may correspond to the range of values between D10 and D84.

The average value of the particle size D84 may be 18.4 nm.

According to the formula described above, the average closest distance between two crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle is from 3 to 100 nm.

Preferably, the average distance between two crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle is from 3 to 80 nm, more preferably from 3 to 70 nm, more preferably from 3 to 50 nm, even more preferably from 5 to 20 nm according to formula (I) of the invention.

The concentration of crosslinkers (C_(XL)) in the first aqueous polymerization solution or in the second aqueous solution may be from 0.001 to 0.5 mol %, more preferably from 0.02 to 0.25 mol %, even more preferably from 0.05 to 0.15 mol %.

The concentration of inorganic solid particles in the water-absorbing polymer particle may be from 0.1 to 8% by weight compared to the total weight of the water absorbent polymer particles in dry state. Preferably, the concentration of inorganic solid particles in the water-absorbing polymer particle is from 0.5 to 3% by weight compared to the total weight of the water absorbent polymer particles in dry state.

The inorganic solid particles that can fit the average closest distance between two neighboring crosslinkers (R_(XL)) in the water-absorbing polymer particle at 20 g/g X-load of the water-absorbing polymer particle may act as barrier particles to the crosslinked polymer network such that the diffusion of the “extractables” outside of the crosslinked polymer network is limited but the influx of fluid through the crosslinked polymer network is not slowed down significantly by the inorganic solid particles.

The water-absorbing polymer particles obtained via the process of the invention may have a relatively low amount of “extractables”.

The level of “extractables” in the water-absorbing polymer particles may be less than 15%, preferably less than 10% and more preferably less than 8% based on the total weight of the water-absorbing polymer particles.

The water-absorbing polymer particles of the invention having a relatively low amount of “extractables” may have good performance properties, especially in terms of effective capacity.

Properties of the Water-Absorbing Polymer Particles

The properties of water-absorbing polymer particles described herein may be characterized in various ways.

The Centrifuge Retention Capacity (CRC) measures the liquid absorbed by the water-absorbing polymer particles for free swelling in excess liquid.

The water-absorbing polymer particles of the invention may have a Centrifuge Retention Capacity (CRC) value of more than 25 g/g, preferably more than 26 g/g, more preferably more than 27 g/g as measured according to EDANA method NWSP 241.0.R2.

The water-absorbing polymer particles of the invention may have a Centrifuge Retention Capacity (CRC) value from 26 up to 50 g/g, or from 27 up to 40 g/g, or from 28 up to 35 g/g, as measured according to EDANA method NWSP 241.0.R2.

The CRC value does not reflect any external pressure apply on the water-absorbing polymer particles. The water-absorbing polymer particles having a high CRC value may be preferred since less water-absorbing polymer particles are needed to facilitate a required overall capacity for liquid absorption.

The Absorption Against Pressure (AAP) of water-absorbing polymer particles corresponds to the capability of the water-absorbing polymer particles to swell against external pressure. The term “external pressure” refers to the pressure applied on the absorbent core of an absorbent article by the wearer when he is seated for example or lay down.

The water-absorbing polymer particles may have a value of Absorption Against Pressure (AAP) of at least 22 g/g, preferably of at least 22.5 g/g, more preferably of at least 23 g/g, even more preferably of at least 23.5 g/g, still preferably of at least 24 g/g according to the Absorption Against Pressure Test Method.

The Absorption Against Pressure Test Method refers to the EDANA method WSP 442.2-02. However, contrary to the pressure specified in EDANA method WSP 442.2-02 (namely 0.3 psi), for the present invention the pressure applied on the sample is 0.7 psi.

The water-absorbing polymer particles may have a relatively high value of Absorption Against Pressure (AAP) in order to allow the water-absorbing polymer particles to swell properly against pressure.

Another parameter to define the properties of water-absorbing polymer particles may be used. It is called the Effective Capacity (EFFC). The Effective Capacity (EFFC) is calculated with the value of Centrifuge Retention Capacity (CRC) (EDANA method NWSP 241.0.R2) and with the value of Absorption Against Pressure (AAP) of the water-absorbing polymer particles. The Effective Capacity represents an average (arithmetic average) of the value of Centrifuge Retention Capacity (CRC) and of the value of Absorption Against Pressure (AAP) of the water-absorbing polymer particles.

The Effective Capacity (EFFC) is calculated via the formula below:

EFFC=(CRC+AAP)/2.

Preferably, the water-absorbing polymer particles have a value of Effective Capacity (EFFC) of at least 24.5 g/g, more preferably of at least 25 g/g, even more preferably of at least 25.5 g/g according to the EFFC test method.

Preferably, the water-absorbing polymer particles have a value of Effective Capacity (EFFC) between 25 g/g and 28 g/g, more preferably between 26 g/g and 28 g/g according to the EFFC test method.

The Absorption Against Pressure (AAP) of water-absorbing polymer particles is an important parameter to measure to the capability of the water-absorbing polymer particles to swell against external pressure. The Centrifuge Retention Capacity of water-absorbing polymer particles is another important parameter to measure the liquid absorbed by the water-absorbing polymer particles for free swelling in excess liquid. In order to have an overview of the situation depending on the pressure applied, an average value of Centrifuge Retention Capacity (CRC) and of the value of Absorption Against Pressure (AAP) of the water-absorbing polymer particles may be useful. That is why the Effective Capacity (EFFC) of the water-absorbing polymer particles is calculated.

The water-absorbing polymer particles may have a permeability at equilibrium expressed as UPM (Urine Permeability Measurement) value of more than 5 UPM, preferably more than 15 UPM, more preferably more than 30 UPM, more preferably more than 50 UPM, or even more preferably more than 70 UPM units according to the UPM test method, where 1 UPM unit is 1×10′ (cm³·s)/g.

Preferably, the water-absorbing polymer particles have a value of UPM of at least 5 UPM units according to the UPM test method.

The UPM value is measured according to the UPM test method described herein. The UPM Test method typically measures the flow resistance of a pre-swollen water-absorbing polymer particles, i.e. the flow resistance is measured at equilibrium. Therefore, such water-absorbing polymer particles having a high UPM value exhibit a high permeability when a significant volume of the absorbent article is already wetted by the liquid exudates. These embodiments exhibit good absorption properties not only at the first gush but also at the subsequent gushes.

Absorbent Articles

A typical disposable absorbent article, in which the water-absorbing polymer particles of the present invention can be used, is placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body and is represented in FIG. 1 to FIG. 5 in the form of a diaper 20.

In more details, FIG. 1 is a plan view of an exemplary diaper 20, in a flat-out state, with portions of the diaper being cut-away to more clearly show the construction of the diaper 20. This diaper 20 is shown for illustration purpose only as the structure of the present invention may be comprised in a wide variety of diapers or other absorbent articles.

As shown in FIGS. 1 and 2, the absorbent article, here a diaper, can comprise a liquid pervious topsheet 24, a liquid impervious backsheet 26, an absorbent core 28 which is positioned between the topsheet 24 and the backsheet 26. The absorbent core 28 can absorb and contain liquid received by the absorbent article and may comprise absorbent materials 60, such as the water-absorbing polymer particles of the present invention 66 and/or cellulose fibers, as well as other absorbent and non-absorbent materials commonly used in absorbent articles (e.g. thermoplastic adhesives immobilizing the water-absorbing polymer particles). The absorbent material and non-absorbent material may be wrapped within a substrate (e.g. one or more nonwovens, tissues etc.) such as by an upper core cover layer 56 facing towards the topsheet and a lower cover layer 58 facing towards the backsheet. Such upper and lower core cover layers may be made of nonwovens, tissues or the like and may be attached to each other continuously or discontinuously, e.g. along their perimeter

The absorbent core may comprise one or more substrate layer(s) (such as nonwoven webs or paper tissue), water-absorbing polymer particles disposed on the one or more substrate layers, and a thermoplastic composition typically disposed on the water-absorbing polymer particles. Typically the thermoplastic composition is a thermoplastic adhesive material. In one embodiment, the thermoplastic adhesive material forms a fibrous layer which is at least partially in contact with the water-absorbing polymer particles on the one or more substrate layers and partially in contact with the one or more substrate layers. Auxiliary adhesive might be deposited on the one or more substrate layers before application of the water-absorbing polymer particles for enhancing adhesion of the water-absorbing polymer particles and/or of the thermoplastic adhesive material to the respective substrate layer(s). The absorbent core may also include one or more cover layer(s) such that the water-absorbing polymer particles are comprised between the one or more substrate layer(s) and the one or more cover layer(s). The one or more substrate layer(s) and the cover layer(s) may comprise or consist of a nonwoven web. The absorbent core may further comprise odor control compounds.

The absorbent core may consist essentially of the one or more substrate layer(s), the water-absorbing polymer particles, the thermoplastic composition, optionally the auxiliary adhesive, optionally the cover layer(s), and optionally odor control compounds.

The absorbent core may also comprise a mixture of water-absorbing polymer particles and airfelt, which may be enwrapped within one or more substrate layers, such as nonwoven webs or paper tissue. Such absorbent cores may comprise from 30% to 95%, or from 50% to 95% of water-absorbing polymer particles by weight of the absorbent material and may comprise from 5% to 70%, or from 5% to 50% of airfelt by weight of the absorbent material (for these percentages, any enwrapping substrate layers are not considered as absorbent material). The absorbent core may also be free of airfelt and may comprise 100% of water-absorbing polymer particles by weight of the absorbent material.

The absorbent core may comprise mixtures of the water-absorbing polymer particles of the present invention and other water-absorbing polymer particles. For example, the absorbent core may comprise at least 70%, or at least 80%, or at least 90% or 100% of water-absorbing polymer particles by weight of the absorbent material, wherein the water-absorbing polymer particles comprise at least 10%, or at least 20% or at least 30% or at least 50% by weight of the water-absorbing polymer particles.

The absorbent articles of the invention, especially diapers and pants, may comprise an acquisition layer 52, a distribution layer 54, or combination of both (all herein collectively referred to as acquisition-distribution system “ADS” 50). The function of the ADS 50 is typically to quickly acquire the fluid and distribute it to the absorbent core in an efficient manner. The ADS may comprise one, two or more layers.

The ADS may be free of water-absorbing polymer particles. The prior art discloses many types of acquisition-distribution systems, see for example WO2000/59430, WO95/10996, U.S. Pat. No. 5,700,254, WO02/067809. However, the water-absorbing polymer particles of the present invention may also be comprised by the ADS.

The function of a distribution layer 54 is to spread the insulting fluid liquid over a larger surface within the article so that the absorbent capacity of the absorbent core can be more efficiently used. Distribution layers may be made of a nonwoven material based on synthetic or cellulosic fibers and having a relatively low density. The distribution layer may typically have an average basis weight of from 30 to 400 g/m², in particular from 80 to 300 g/m².

The distribution layer may for example comprise at least 50%, or 60%, or 70%, or 80%, or 90% by weight of cross-linked cellulose fibers. The cross-linked cellulosic fibers may be crimped, twisted, or curled, or a combination thereof including crimped, twisted, and curled. The cross-linked cellulosic fibers provide higher resilience and therefore higher resistance to the first absorbent layer against the compression in the product packaging or in use conditions, e.g. under baby weight. This provides the core with a relatively high void volume, permeability and liquid absorption, and hence reduced leakage and improved dryness.

The distribution layer comprising cross-linked cellulose fibers, may comprise other fibers, but this layer may advantageously comprise at least 50%, or 60%, or 70%, or 80%, or 90% or even up to 100%, by weight of the layer, of cross-linked cellulose fibers. Examples of such mixed layer of cross-linked cellulose fibers may comprise 70% by weight of chemically cross-linked cellulose fibers, 10% by weight polyester (PET) fibers, and 20% by weight untreated pulp fibers. In another example, the layer of cross-linked cellulose fibers may comprise 70% by weight chemically cross-linked cellulose fibers, 20% by weight lyocell fibers, and 10% by weight PET fibers. In another example, the layer may comprise 68% by weight chemically cross-linked cellulose fibers, 16% by weight untreated pulp fibers, and 16% by weight PET fibers.

The absorbent article 20 may further comprise an acquisition layer 52, whose function is to quickly acquire the fluid away from the topsheet so as to provide a good dryness for the wearer. The acquisition layer 52 is typically placed directly under the topsheet and below the distribution layer. The acquisition layer may typically be or comprise a non-woven material, for example a SMS or SMMS material, comprising a spunbonded, a melt-blown and a further spunbonded layer or alternatively a carded chemical-bonded nonwoven. The non-woven material may in particular be latex bonded. Exemplary upper acquisition layers 52 are disclosed in U.S. Pat. No. 7,786,341. Carded, resin-bonded nonwovens may be used, in particular where the fibers used are solid round or round and hollow PET staple fibers (such as a 50/50 or 40/60 mix of 6 denier and 9 denier fibers). An exemplary binder is a butadiene/styrene latex.

The acquisition layer 52 may be stabilized by a latex binder, for example a styrene-butadiene latex binder (SB latex). Processes for obtaining such lattices are known, for example, from EP 149 880 (Kwok) and US 2003/0105190 (Diehl et al.). The binder may be present in the acquisition layer 52 in excess of 12%, 14% or 16% by weight, but may be present by not more than 30%, or not more than 25% by weight of the acquisition layer. SB latex is available under the trade name GENFLO™ 3160 (OMNOVA Solutions Inc.; Akron, Ohio).

A further acquisition layer may be used in addition to a first acquisition layer described above. For example a tissue layer may be placed between the first acquisition layer and the distribution layer. The tissue may have enhanced capillarity distribution properties compared to the acquisition layer described above. The tissue and the first acquisition layer may be of the same size or may be of different size, for example the tissue layer may extend further in the back of the absorbent article than the first acquisition layer. An example of hydrophilic tissue is a 13 to 15 gsm high wet strength made of cellulose fibers from supplier Havix.

The diaper may also comprise elasticized leg cuffs 32 and barrier leg cuffs 34, which provide improved containment of liquids and other body exudates especially in the area of the leg openings. Usually each leg cuffs 32 and barrier cuffs 34 will comprise one or more elastic string 33 and 35, represented in exaggerated form on FIGS. 1 and 2. Moreover, the diaper 20 may comprise other features such as back ears 40, front ears 46 and/or barrier cuffs 34 attached to form the composite diaper structure. The diaper may further comprise a fastening system, such as an adhesive fastening system or a mechanical fastening system (e.g. a hook and loop fastening system), which can comprise tape tabs 42, such as adhesive tape tabs or tape tabs comprising hook elements, cooperating with a landing zone 44 (e.g. a nonwoven web providing loops in a hook and loop fastening system). Further, the diaper may comprise other elements, such as a back elastic waist feature and a front elastic waist feature, side panels or a lotion application.

The diaper 20 as shown in FIGS. 1 and 2 can be notionally divided in a first waist region 36, a second waist region 38 opposed to the first waist region 36 and a crotch region 37 located between the first waist region 36 and the second waist region 38. The longitudinal centerline 80 is the imaginary line separating the diaper along its length in two equal halves. The transversal centerline 90 is the imagery line perpendicular to the longitudinal line 80 in the plane of the flattened out diaper and going through the middle of the length of the diaper. The periphery of the diaper 20 is defined by the outer edges of the diaper 20. The longitudinal edges of the diaper may run generally parallel to the longitudinal centerline 80 of the diaper 20 and the end edges run between the longitudinal edges generally parallel to the transversal centerline 90 of the diaper 20.

Area(s) 29 substantially free of absorbent material and channels 29′

As shown in FIG. 3, the absorbent core 28 may comprise one or more area(s) 29 which is/are substantially free of absorbent material. By “substantially free” it is meant that in each of these areas the basis weight of the absorbent material is less than 25%, in particular less than 20%, less than 10%, of the average basis weight of the absorbent material in the rest of the core. In particular there can be no absorbent material in these areas. Minimal amount such as involuntary contaminations with absorbent material that may occur during the making process are not considered as absorbent material. The areas 29 are advantageously surrounded by the absorbent material, when seen in the plane of the core, which means that the area(s) 29 does not extend to any of the edge of the deposition area 8 of the absorbent material.

The upper core cover layer 56 is attached to the lower cover layer 58 by core wrap bond(s) 27 through these area(s) 29 substantially free of absorbent material. As shown in FIG. 4 and FIG. 5, when the absorbent material swells upon absorbing a liquid, the core wrap bond remains at least initially attached in the substantially material free area(s) 29. The absorbent material swells in the rest of the core when it absorbs a liquid, so that the core wrap forms one or more channel(s) 29′ along the area(s) 29 substantially free of absorbent material comprising the core wrap bond 27. These channels 29′ are three dimensional and can serve to distribute an insulting fluid along their length to a wider area of the core. This may provide a quicker fluid acquisition speed and a better utilization of the absorbent capacity of the core. The channels 29′ can also provide a deformation of an overlying layer such as a fibrous layer 54 and provide corresponding ditches 29 in the overlying layer. It is not excluded that the absorbent core may comprise other area(s) substantially free of absorbent material but without a core wrap bond, but these non-bonded areas will typically not form a channel when wet.

The upper core cover layer 56 and the lower cover layer 58 may be attached together continuously along the area(s) 29 substantially free of absorbent material, but the core wrap bond 27 may also be discontinuous (intermittent) such as series of point bonds. Typically, an adhesive can be used to attach the top side to the bottom of the core wrap, but it is possible to bond via other known attachment means, such as pressure bonding, ultrasonic bonding or heat bonding or combination thereof. The attachment of the top side and bottom side of the core wrap may be provided by one or more adhesive material, in particular one or more layers of auxiliary glue and/or one or more layers of fibrous adhesive material, if present in the core, as indicated below. These glues may therefore serve the dual function of immobilizing the absorbent material and attach the top side and the bottom side of the core together.

The following examples of the shape and size of the areas 29 substantially free of absorbent material are not limiting. In general, the core wrap bond 27 may have the same outline but be slightly smaller than the areas 29 due to the tolerance required in some manufacturing process. The substantially material free area(s) 29 may be present within the crotch region of the article, in particular at least at the same longitudinal level as the crotch point C, as represented in FIG. 3 by the two longitudinally extending areas substantially free of absorbent material 29. The absorbent core 28 may also comprise more than two substantially absorbent material free area(s), for example at least 3, or at least 4 or at least 5 or at least 6. The absorbent core may comprise one or more pairs of areas substantially free of absorbent material symmetrically arranged relative to the longitudinal axis 80. Shorter area(s) substantially free of absorbent material may also be present, for example in the back region or the front region of the core, as seen for example in the Figures of WO2012/170778.

The area(s) 29 substantially free of absorbent material may extend substantially longitudinally, which means typically that each area extends more in the longitudinal direction than in the transverse direction, and typically at least twice as much in the longitudinal direction than in the transverse direction (as measured after projection on the respective axis). The area(s) 29 substantially free of absorbent material may have a length projected on the longitudinal axis 80 of the core that is at least 10% of the length of the absorbent core, in particular from 20% to 80%. It may be advantageous that at least some or all of the area(s) 29 are not completely or substantially completely transversely oriented channels in the core.

The area(s) 29 substantially free of absorbent material may be completely oriented longitudinally and parallel to the longitudinal axis but also may be curved. In particular some or all these area(s), in particular these area(s) present in the crotch region, may be concave towards the longitudinal axis 80, as for example represented in FIG. 3 for the pair of channels 29′. The radius of curvature may typically be at least equal (and preferably at least 1.5 or at least 2.0 times this average transverse dimension) to the average transverse dimension of the absorbent material deposition area 8; and also straight but under an angle of (e.g. from 5°) up to 30°, or for example up to 20°, or up to 10° with a line parallel to the longitudinal axis. The radius of curvature may be constant for a substantially absorbent material free area(s), or may vary along its length. This may also includes area(s) substantially free of absorbent material with an angle therein, provided said angle between two parts of a channel is at least 120°, preferably at least 150°; and in any of these cases, provided the longitudinal extension of the area is more than the transverse extension. These area(s) may also be branched, for example a central substantially material free area superposed with the longitudinal axis in the crotch region which branches towards the back and/or towards the front of the article.

In some embodiments, there is no area(s) substantially free of absorbent material that coincides with the longitudinal axis 80 of the core. When present as one ore symmetrical pair(s) relative to the longitudinal axis, the area(s) substantially free of absorbent material may be spaced apart from one another over their whole longitudinal dimension. The smallest spacing distance may be for example at least 5 mm, or at least 10 mm, or at least 16 mm.

Furthermore, in order to reduce the risk of fluid leakages, the area(s) substantially free of absorbent material may advantageously not extend up to any of the edges of the absorbent material deposition area 8, and are therefore surrounded by and fully encompassed within the absorbent material deposition area 8 of the core. Typically, the smallest distance between an area(s) substantially free of absorbent material and the closest edge of the absorbent material deposition area is at least 5 mm.

The area(s) substantially free of absorbent material may have a width Wc along at least part of its length which is at least 2 mm, or at least 3 mm or at least 4 mm, up to for example 20 mm, or 16 mm or 12 mm. The width Wc of the area(s) substantially free of absorbent material may be constant through substantially its whole length or may vary along its length.

The channels 29′ in the absorbent core start forming when the absorbent material absorbs a liquid such as urine and starts swelling. As the core absorbs more liquid, the depressions within the absorbent core formed by channels will become deeper and more apparent to the eye and the touch. It is possible to create a sufficiently strong core wrap bond combined with a relatively low amount of water-absorbing polymer particles so that the channels remain permanent until complete saturation of the absorbent material. On the other hand, the core wrap bonds may in some cases also restrict the swelling of the absorbent material when the core is substantially loaded.

Initially, the core wrap bond(s) may be designed to be closed and to increase the pressure in the areas adjacent to the core wrap bond(s). At some point, the core wrap bond 27 may also be designed to open in a controlled manner when exposed to a large amount of fluid.

Test Methods

-   -   Urine Permeability Measurement (UPM) Test Method

Lab Conditions:

This test has to be performed in a climate conditioned room at standard conditions of 23° C.±2° C. temperature and 45%±10% relative humidity.

Urine Permeability Measurement System

This method determined the permeability of a swollen hydrogel layer 1318. The equipment used for this method is described below. This method is closely related to the SFC (Salt Flow Conductivity or Saline Flow Conductivity) test method of the prior art.

FIG. 6 shows permeability measurement system 1000 set-up with the constant hydrostatic head reservoir 1014, open-ended tube for air admittance 1010, stoppered vent for refilling 1012, laboratory reck 1016, delivery tube 1018 with flexible tube 1045 with Tygon tube nozzle 1044, stopcock 1020, cover plate 1047 and supporting ring 1040, receiving vessel 1024, balance 1026 and piston/cylinder assembly 1028.

FIG. 7 shows the piston/cylinder assembly 1028 comprising a metal weight 1112, piston shaft 1114, piston head 1118, lid 1116, and cylinder 1120. The cylinder 1120 is made of transparent polycarbonate (e.g., Lexan®) and has an inner diameter p of 6.00 cm (area=28.27 cm²) with inner cylinder walls 1150 which are smooth. The bottom 1148 of the cylinder 1120 is faced with a stainless-steel screen cloth (ISO 9044 Material 1.4401, mesh size 0.038 mm, wire diameter 0.025 mm) (not shown) that is bi-axially stretched to tautness prior to attachment to the bottom 1148 of the cylinder 1120. The piston shaft 1114 is made of transparent polycarbonate (e.g., Lexan®) and has an overall length q of approximately 127 mm. A middle portion 1126 of the piston shaft 1114 has a diameter r of 22.15 (±0.02) mm. An upper portion 1128 of the piston shaft 1114 has a diameters of 15.8 mm, forming a shoulder 1124. A lower portion 1146 of the piston shaft 1114 has a diameter t of approximately ⅝ inch (15.9 mm) and is threaded to screw firmly into the center hole 1218 (see FIG. 8) of the piston head 1118. The piston head 1118 is perforated, made of transparent polycarbonate (e.g., Lexan®), and is also screened with a stretched stainless-steel screen cloth (ISO 9044 Material 1.4401, mesh size 0.038 mm, wire diameter 0.025 mm) (not shown). The weight 1112 is stainless steel, has a center bore 1130, slides onto the upper portion 1128 of piston shaft 1114 and rests on the shoulder 1124. The combined weight of the piston head 1118, piston shaft 1114 and weight 1112 is 596 g (±6 g), which corresponds to 0.30 psi over the inner area of the cylinder 1120. The combined weight may be adjusted by drilling a blind hole down a central axis 1132 of the piston shaft 1114 to remove material and/or provide a cavity to add weight. The cylinder lid 1116 has a first lid opening 1134 in its center for vertically aligning the piston shaft 1114 and a second lid opening 1136 near the edge 1138 for introducing fluid from the constant hydrostatic head reservoir 1014 into the cylinder 1120.

A first linear index mark (not shown) is scribed radially along the upper surface 1152 of the weight 1112, the first linear index mark being transverse to the central axis 1132 of the piston shaft 1114. A corresponding second linear index mark (not shown) is scribed radially along the top surface 1160 of the piston shaft 1114, the second linear index mark being transverse to the central axis 1132 of the piston shaft 1114. A corresponding third linear index mark (not shown) is scribed along the middle portion 1126 of the piston shaft 1114, the third linear index mark being parallel with the central axis 1132 of the piston shaft 1114. A corresponding fourth linear index mark (not shown) is scribed radially along the upper surface 1140 of the cylinder lid 1116, the fourth linear index mark being transverse to the central axis 1132 of the piston shaft 1114. Further, a corresponding fifth linear index mark (not shown) is scribed along a lip 1154 of the cylinder lid 1116, the fifth linear index mark being parallel with the central axis 1132 of the piston shaft 1114. A corresponding sixth linear index mark (not shown) is scribed along the outer cylinder wall 1142, the sixth linear index mark being parallel with the central axis 1132 of the piston shaft 1114. Alignment of the first, second, third, fourth, fifth, and sixth linear index marks allows for the weight 1112, piston shaft 1114, cylinder lid 1116, and cylinder 1120 to be repositioned with the same orientation relative to one another for each measurement.

The cylinder 1120 specification details are:

Outer diameter u of the Cylinder 1120: 70.35 mm (±0.05 mm) Inner diameter p of the Cylinder 1120: 60.0 mm (±0.05 mm) Height v of the Cylinder 1120: 60.5 mm. Cylinder height must not be lower than 55.0 mm!

The cylinder lid 1116 specification details are:

Outer diameter w of cylinder lid 1116: 76.05 mm (±0.05 mm) Inner diameter x of cylinder lid 1116: 70.5 mm (±0.05 mm) Thickness y of cylinder lid 1116 including lip 1154: 12.7 mm Thickness z of cylinder lid 1116 without lip 1154: 6.35 mm Diameter a of first lid opening 1134: 22.25 mm (±0.02 mm) Diameter b of second lid opening 1136: 12.7 mm (±0.1 mm) Distance between centers of first and second lid openings 1134 and 1136: 23.5 mm

The weight 1112 specification details are:

Outer diameter c: 50.0 mm Diameter d of center bore 1130: 16.0 mm

Height e: 39.0 mm

The piston head 1118 specification details are:

Diameter f: 59.7 mm (±0.05 mm)

Height g: 16.5 mm. Piston head height must not be less than 15.0 mm. Outer holes 1214 (14 total) with a 9.30 (±0.25) mm diameter h, outer holes 1214 equally spaced with centers being 23.9 mm from the center of center hole 1218. Inner holes 1216 (7 total) with a 9.30 (±0.25) mm diameter i, inner holes 1216 equally spaced with centers being 13.4 mm from the center of center hole 1218. Center hole 1218 has a diameter j of approximately ⅝ inches (15.9 mm) and is threaded to accept a lower portion 1146 of piston shaft 1114.

Prior to use, the stainless steel screens (not shown) of the piston head 1118 and cylinder 1120 should be inspected for clogging, holes or over-stretching and replaced when necessary. A urine permeability measurement apparatus with damaged screen can deliver erroneous UPM results, and must not be used until the screen has been replaced.

A 5.00 cm mark 1156 is scribed on the cylinder 1120 at a height k of 5.00 cm (±0.05 cm) above the screen (not shown) attached to the bottom 1148 of the cylinder 1120. This marks the fluid level to be maintained during the analysis. Maintenance of correct and constant fluid level (hydrostatic pressure) is critical for measurement accuracy.

A constant hydrostatic head reservoir 1014 is used to deliver salt solution 1032 to the cylinder 1120 and to maintain the level of salt solution 1032 at a height k of 5.00 cm above the screen (not shown) attached to the bottom 1148 of the cylinder 1120. The bottom 1034 of the air-intake tube 1010 is positioned so as to maintain the salt solution 1032 level in the cylinder 1120 at the required 5.00 cm height k during the measurement, i.e., bottom 1034 of the air tube 1010 is in approximately same plane 1038 as the 5.00 cm mark 1156 on the cylinder 1120 as it sits on the cover plate 1047 and supporting ring 1040 (with circular inner opening of not less than 64 mm diameter) above the receiving vessel 1024.

The cover plate 1047 and supporting ring 1040 are parts as used in the equipment used for the method “K(t) Test Method (Dynamic Effective Permeability and Uptake Kinetics Measurement Test method)” as described in EP 2 535 027 A1 and is called “Zeitabhängiger Durchlässigkeitsprüfstand” or “Time Dependent Permeability Tester”, Equipment No. 03-080578 and is commercially available at BRAUN GmbH, Frankfurter Str. 145, 61476 Kronberg, Germany. Upon request, detailed technical drawings are also available.

Proper height alignment of the air-intake tube 1010 and the 5.00 cm mark 1156 on the cylinder 1120 is critical to the analysis. A suitable reservoir 1014 consists of a jar 1030 containing: a horizontally oriented L-shaped delivery tube 1018 connected to a flexible tube 1045 (e.g. Tygon tube, capable to connect nozzle and reservoir outlet) and to a Tygon tube nozzle 1044 (inner diameter at least 6.0 mm, length appr. 5.0 cm) for fluid delivery, a vertically oriented open-ended tube 1010 for admitting air at a fixed height within the constant hydrostatic head reservoir 1014, and a stoppered vent 1012 for re-filling the constant hydrostatic head reservoir 1014. Tube 1010 has an internal diameter of approximately 12 mm, but not less than 10.5 mm. The delivery tube 1018, positioned near the bottom 1042 of the constant hydrostatic head reservoir 1014, contains a stopcock 1020 for starting/stopping the delivery of salt solution 1032. The outlet 1044 of the delivery flexible tube 1045 is dimensioned (e.g. outer diameter 10 mm) to be inserted through the second lid opening 1136 in the cylinder lid 1116, with its end positioned below the surface of the salt solution 1032 in the cylinder 1120 (after the 5.00 cm height of the salt solution 1032 is attained in the cylinder 1120). The air-intake tube 1010 is held in place with an o-ring collar 1049. The constant hydrostatic head reservoir 1014 can be positioned on a laboratory reck 1016 at a suitable height relative to that of the cylinder 1120. The components of the constant hydrostatic head reservoir 1014 are sized so as to rapidly fill the cylinder 1120 to the required height (i.e., hydrostatic head) and maintain this height for the duration of the measurement. The constant hydrostatic head reservoir 1014 must be capable of delivering salt solution 1032 at a flow rate of at least 2.6 g/sec for at least 10 minutes.

The piston/cylinder assembly 1028 is positioned on the supporting ring 1040 in the cover plate 1047 or suitable alternative rigid stand. The salt solution 1032 passing through the piston/cylinder assembly 1028 containing the swollen hydrogel layer 1318 is collected in a receiving vessel 1024, positioned below (but not in contact with) the piston/cylinder assembly 1028.

The receiving vessel 1024 is positioned on the balance 1026 which is accurate to at least 0.001 g. The digital output of the balance 1026 is connected to a computerized data acquisition system 1048.

Preparation of Reagents (not Illustrated)

Jayco Synthetic Urine (JSU) 1312 (see FIG. 9) is used for a swelling phase (see UPM Procedure below) and 0.118 M Sodium Chloride (NaCl) Solution 1032 is used for a flow phase (see UPM Procedure below). The following preparations are referred to a standard 1 liter volume. For preparation of volumes other than 1 liter, all quantities are scaled accordingly.

JSU: A 1 L volumetric flask is filled with distilled water to 80% of its volume, and a magnetic stir bar is placed in the flask. Separately, using a weighing paper or beaker the following amounts of dry ingredients are weighed to within ±0.01 g using an analytical balance and are added quantitatively to the volumetric flask in the same order as listed below. The solution is stirred on a suitable stir plate until all the solids are dissolved, the stir bar is removed, and the solution diluted to 1 L volume with distilled water. A stir bar is again inserted, and the solution stirred on a stirring plate for a few minutes more.

Quantities of salts to make 1 liter of Jayco Synthetic Urine:

Potassium Chloride (KCl) 2.00 g Sodium Sulfate (Na2SO4) 2.00 g

Ammonium dihydrogen phosphate (NH4H2PO4) 0.85 g Ammonium phosphate, dibasic ((NH4)2HPO4) 0.15 g Calcium chloride (CaCl2) 0.19 g-[or hydrated calcium chloride (CaCl2.2H2O) 0.25 g] Magnesium chloride (MgCl2) 0.23 g-[or hydrated magnesium chloride (MgCl2.6H2O) 0.50 g]

To make the preparation faster, potassium chloride, sodium sulfate, ammonium dihydrogen phosphate, ammonium phosphate (dibasic) and magnesium chloride (or hydrated magnesium chloride) are combined and dissolved in the 80% of distilled water in the 1 L volumetric flask. Calcium chloride (or hydrated calcium chloride) is dissolved separately in approximately 50 ml distilled water (e.g. in a glass beaker) and the calcium chloride solution is transferred to the 1 L volumetric flask after the other salts are completely dissolved therein. Afterwards, distilled water is added to 1 L (1000 ml 0.4 ml) and the solution is stirred for a few minutes more. Jayco synthetic urine may be stored in a clean plastic container for 10 days. The solution should not be used if it becomes cloudy. 0.118 M Sodium Chloride (NaCl) Solution: 0.118 M Sodium Chloride is used as salt solution 1032. Using a weighing paper or beaker 6.90 g (±0.01 g) of sodium chloride is weighed and quantitatively transferred into a 1 L volumetric flask (1000 ml±0.4 ml); and the flask is filled to volume with distilled water. A stir bar is added and the solution is mixed on a stirring plate until all the solids are dissolved.

The conductivity of the prepared Jayco solution must be in the range of appr. 7.48-7.72 mS/cm and of the prepared 0.118 M Sodium Chloride (NaCl) Solution in the range of appr. 12.34-12.66 mS/cm (e.g. measured via COND 70 INSTRUMENT without CELL, #50010522, equipped with Cell VPT51-01 C=0.1 from xs instruments or via LF 320/Set, #300243 equipped with TetraCon 325 from WTW or COND 330i, #02420059 equipped with TetraCon 325 from WTW). The surface tension of each of the solutions must be in the range of 71-75 mN/m (e.g. measured via tensiometer K100 from Kruess with Pt plate).

Test Preparation

Using a solid reference cylinder weight (not shown) (50 mm diameter; 128 mm height), a caliper gauge (not shown) (measurement range 25 mm, accurate to 0.01 mm, piston pressure max. 50 g; e.g. Mitutoyo Digimatic Height Gage) is set to read zero. This operation is conveniently performed on a smooth and level bench (not shown) of at least approximately 11.5 cm×15 cm. The piston/cylinder assembly 1028 without superabsorbent polymer particles is positioned under the caliper gauge (not shown) and a reading, L1, is recorded to the nearest 0.01 mm.

The constant hydrostatic head reservoir 1014 is filled with salt solution 1032. The bottom 1034 of the air-intake tube 1010 is positioned so as to maintain the top part (not shown) of the liquid meniscus (not shown) in the cylinder 1120 at the 5.00 cm mark 1156 during the measurement. Proper height alignment of the air-intake tube 1010 at the 5.00 cm mark 1156 on the cylinder 1120 is critical to the analysis.

The receiving vessel 1024 is placed on the balance 1026 and the digital output of the balance 1026 is connected to a computerized data acquisition system 1048. The cover plate 1047 with the supporting ring 1040 is positioned above the receiving vessel 1024.

UPM Procedure

1.5 g (±0.05 g) of superabsorbent polymer particles is weighed onto a suitable weighing paper or weighing aid using an analytical balance. The moisture content of the superabsorbent polymer particles is measured according to the Edana Moisture Content Test Method NWSP 230.0.R2 (15) or via a Moisture Analyzer (HX204 from Mettler Toledo, drying temperature 130° C., starting superabsorber weight 3.0 g (±0.5 g), stop criterion 1 mg/140 s). If the moisture content of the superabsorbent polymer particles is greater than 3 wt %, then the superabsorbent polymer particles are dried to a moisture level of <3 wt %, e.g. in an oven at 105° C. for 3 h or e.g. at 120° C. for 2 h. Agglomerated superabsorbent polymer particles are dried if moisture level is greater than 5 wt %, e.g. in an oven at 105° C. for 3 h or e.g. at 120° C. for 2 h.

The empty cylinder 1120 is placed on a level benchtop 1046 (not shown) and the superabsorbent polymer particles are quantitatively transferred into the cylinder 1120. The superabsorbent polymer particles are evenly dispersed on the screen (not shown) attached to the bottom 1148 of the cylinder 1120 while rotating the cylinder 1120, e.g. aided by a (manual or electrical) turn table (e.g. petriturn-E or petriturn-M from Schuett). It is important to have an even distribution of particles on the screen (not shown) attached to the bottom 1148 of the cylinder 1120 to obtain the highest precision result. After the superabsorbent polymer particles have been evenly distributed on the screen (not shown) attached to the bottom 1148 of the cylinder 1120 particles must not adhere to the inner cylinder walls 1150. The piston shaft 1114 is inserted through the first lid opening 1134, with the lip 1154 of the lid 1116 facing towards the piston head 1118. The piston head 1118 is carefully inserted into the cylinder 1120 to a depth of a few centimeters. The lid 1116 is then placed onto the upper rim 1144 of the cylinder 1120 while taking care to keep the piston head 1118 away from the superabsorbent polymer particles. The weight 1112 is positioned on the upper portion 1128 of the piston shaft 1114 so that it rests on the shoulder 1124 such that the first and second linear index marks are aligned. The lid 1116 and piston shaft 1126 are then carefully rotated so as to align the third, fourth, fifth, and sixth linear index marks are then aligned with the first and the second linear index marks. The piston head 1118 (via the piston shaft 1114) is then gently lowered to rest on the dry superabsorbent polymer particles. Proper seating of the lid 1116 prevents binding and assures an even distribution of the weight on the hydrogel layer 1318.

Swelling Phase:

A fritted disc of at least 8 cm diameter (e.g. 8-9 cm diameter) and at least 5.0 mm thickness (e.g. 5-7 mm thickness) with porosity “coarse” or “extra coarse” (e.g. Chemglass Inc. # CG 201-51, coarse porosity; or e.g. Robu 1680 with porosity 0) 1310 is placed in a wide flat-bottomed Petri dish 1314 and JSU 1312 is added by pouring JSU 1312 onto the center of the fritted disc 1310 until JSU 1312 reaches the top surface 1316 of the fritted disc 1310. The JSU height must not exceed the height of the fritted disc 1310. It is important to avoid any air or gas bubbles entrapped in or underneath the fritted disc 1310.

The entire piston/cylinder assembly 1028 is lifted and placed on the fritted disc 1310 in the Petri dish 1314. JSU 1312 from the Petri dish 1314 passes through the fritted disc 1310 and is absorbed by the superabsorbent polymer particles (not shown) to form a hydrogel layer 1318. The JSU 1312 available in the Petri dish 1314 should be enough for all the swelling phase. If needed, more JSU 1312 may be added to the Petri dish 1314 during the hydration period to keep the JSU 1312 level at the top surface 1316 of the fritted disc 1310. After a period of 60 minutes, the piston/cylinder assembly 1028 is removed from the fritted disc 1310, taking care to ensure the hydrogel layer 1318 does not lose JSU 1312 or take in air during this procedure. The piston/cylinder assembly 1028 is placed under the caliper gauge (not shown) and a reading, L2, is recorded to the nearest 0.01 mm. If the reading changes with time, only the initial value is recorded. The thickness of the hydrogel layer 1318, L0 is determined from L2−L1 to the nearest 0.1 mm.

The piston/cylinder assembly 1028 is transferred to the supporting ring 1040 in the cover plate 1047. The constant hydrostatic head reservoir 1014 is positioned such that the delivery tube nozzle 1044 is placed through the second lid opening 1136. The measurement is initiated in the following sequence:

-   -   a) The stopcock 1020 of the constant hydrostatic head reservoir         1014 is opened to permit the salt solution 1032 to reach the         5.00 cm mark 1156 on the cylinder 1120. This salt solution 1032         level should be obtained within 10 seconds of opening the         stopcock 1020.     -   b) Once 5.00 cm of salt solution 1032 is attained, the data         collection program is initiated.

With the aid of a computer 1048 attached to the balance 1026, the quantity g (in g to accuracy of 0.001 g) of salt solution 1032 passing through the hydrogel layer 1318 is recorded at intervals of 20 seconds for a time period of 10 minutes. At the end of 10 minutes, the stopcock 1020 on the constant hydrostatic head reservoir 1014 is closed.

The data from 60 seconds to the end of the experiment are used in the UPM calculation. The data collected prior to 60 seconds are not included in the calculation.

For each time period of 20 seconds (time t_((i-1)) to t_(i)) after the initial 60 seconds of the experiment, the respective flow rate Fs_((t)) (in g/s) and the respective mid-point of the time t_((1/2)t) (in s) is calculated according to the following formulas:

$\begin{matrix} {{Fs}_{(t)} = {{\frac{\left( {g_{({i - 1})} - g_{(i)}} \right)}{\left( {t_{({i - 1})} - t_{(i)}} \right)}\mspace{14mu} {and}\mspace{14mu} t_{{({1\text{/}2})}_{t}}} = \frac{\left( {t_{({i - 1})} + t_{(i)}} \right)}{2}}} & ({XII}) \end{matrix}$

The flow rate Fs_((t)) of each time interval (t_((i-1)) to t_(i)) is plotted versus the mid-point of the time t_((1/2)t) of the time interval (t_((i-1)) to t_(i)). The intercept is calculated as Fs(t=0).

Calculation of the Intercept:

The intercept is calculated via a best-fit regression line, e.g. as following: the equation for the intercept of the regression line, a, is:

a=y _(AVG) −b·x _(AVG)  (XIII)

where the slope, b, is calculated as:

$\begin{matrix} {b = \frac{{\Sigma \left( {x - x_{AVG}} \right)} \cdot \left( {y - y_{AVG}} \right)}{{\Sigma \left( {x - x_{AVG}} \right)}^{2}}} & ({XIV}) \end{matrix}$

and where x_(AVG) and y_(AVG) are the sample means AVERAGE of the known x's and AVERAGE of the known_y's, respectively.

Calculation of Urine Permeability Measurement Q:

The intercept Fs(t=0) is used to calculate Q according to the following formula:

$\begin{matrix} {Q = \frac{{F_{s}\left( {t = 0} \right)} \cdot L_{0}}{{\rho \cdot A \cdot \Delta}\; P}} & ({XV}) \end{matrix}$

where the flow rate Fs(t=0) is given in g/s, L₀ is the initial thickness of the hydrogel layer 1318 in cm, ρ is the density of the salt solution 1032 in g/cm³ (e.g. 1.003 g/cm³ at room temperature). A (from the equation above) is the area of the hydrogel layer 1318 in cm² (e.g. 28.27 cm²), ΔP is the hydrostatic pressure in dyne/cm² (e.g. 4920 dyne/cm²), and the Urine Permeability Measurement, Q, is in units of cm³ sec/g. The average of three determinations should be reported.

TABLE 1 Variable Description Unit g_(i) Mass of salt solution 1032 flown through the swollen gel layer g (recorded by the balance) at the time t_(i) (accuracy 0.001 g) t_(i) Time point (every 20 s) s t_((1/2)t) Mid-point of time for the respective time interval t_(i−1) to t_(i) s Fs_(t) Flow Rate at the time interval t_(i−1) to t_(i) g/s Fs (t = 0) Intercept flow rate at t = 0 s from the plot of the flow rate Fs(t) vs. g/s the mid-point of time t_((1/2)t) . L₀ Thickness of the swollen gel layer (swollen with JSU 1312) before cm the salt solution 1032 flows through the gel layer. ρ Density of the salt solution 1032 (1.003 g/cm³) g/cm³ A Area of the swollen gel layer (28.27 cm²) cm² ΔP Hydrostatic pressure across the gel layer (4920 dyne/cm²) dyne/cm² Q Urine Permeability Measurement cm³ * sec/g

The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.”

Every document cited herein, including any cross referenced or related patent or application and any patent application or patent to which this application claims priority or benefit thereof, is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.

While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention. 

What is claimed is:
 1. A method for making water-absorbing polymer particles by providing: a first aqueous polymerization solution comprising crosslinkers, polymerizable monomers and/or oligomers and inorganic solid particles, wherein the weight ratio of inorganic solid particles based on the dry solid content of the first aqueous polymerization solution is from 0 to 0.5%, a second aqueous solution comprising inorganic solid particles, crosslinkers and either polymerizable monomers and/or oligomers, or crosslinkable polymers wherein the weight ratio of inorganic solid particles based on the dry solid content of the second aqueous solution is more than 0.5%, and wherein the average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution for a specific X-load of a water-absorbing polymer particle is calculated via the formula below: $\begin{matrix} {{Rxl} = \left( \frac{\left( {\frac{1}{{rho}_{—}{dry}} + \frac{x_{—}L}{{rho}_{—}{liq}}} \right)}{N_{A} \cdot {\sum\limits_{i}\frac{w_{—}{xl}_{i}}{{Mr}_{—}{CXL}_{i}}}} \right)^{\frac{1}{3}}} & (I) \end{matrix}$ with x_L corresponding to the amount of liquid absorbed in the water-absorbing polymer particle in g liq/g water-absorbing polymer particle, rho_liq corresponding to the density at room temperature of the fluid that swells the water-absorbing polymer particle (generally saline of 0.9% w NaCl) in g/cm³, rho_dry corresponding to the true density of the dry water-absorbing polymer particle in g/cm³, Mr_CXL corresponding to the molar mass of the crosslinkers from the first aqueous polymerization solution or from the second aqueous solution, respectively, in g/mol, w_xl corresponding to the weight ratio of crosslinkers in dry water-absorbing polymer particle from the first aqueous polymerization solution or from the second aqueous solution, respectively, N_(A) corresponding to the Avogadro's number in mol⁻¹, wherein the average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle obtained via the formula above is at least as high as an average size of the inorganic solid particles or higher than an average size of the inorganic solid particles in the respective solution, and wherein the average closest distance between two neighboring crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle is from 3 to 100 nm, the method comprising the steps of: a) polymerizing the first aqueous polymerization solution to obtain precursor water-absorbing polymer particles, b) mixing the precursor water-absorbing polymer particles with the second aqueous solution, and c) polymerizing the mixed solution if the mixed solution comprises polymerizable monomers and/or oligomers or crosslinking the mixed solution if the mixed solution comprises crosslinkable polymers to obtain the water-absorbing polymer particles.
 2. The method according to claim 1, wherein the average closest distance between two crosslinkers (R_(XL)) from the first aqueous polymerization solution or from the second aqueous solution at 20 g/g X-load of the water-absorbing polymer particle is from 3 to 50 nm.
 3. The method according to claim 1, wherein the concentration of crosslinkers (C_(XL)) in the first aqueous polymerization solution or in the second aqueous solution is from 0.01 to 0.5 mol %.
 4. The method according to claim 1, wherein the average range of size of the inorganic solid particles in the first aqueous polymerization solution or in the second aqueous solution is from 3 to 100 nm.
 5. The method according to claim 1, wherein the crosslinkers from the first aqueous polymerization solution comprise acrylate or acrylamide groups.
 6. The method according to claim 1, wherein the crosslinkers from the second aqueous solution are a first type of crosslinkers comprising acrylate or acrylamide groups.
 7. The method according to claim 1, wherein the crosslinkers from the second aqueous solution are a second type of crosslinkers being amide acetals, carbamic esters, polyhydric alcohols like diols or polyols, cyclic carbonates, bisoxazolines, epoxides or glycidyl ether.
 8. The method according to claim 1, wherein the polymerizable monomers and/or oligomers from the first aqueous polymerization solution or from the second aqueous solution are selected from the group consisting of ethylenically unsaturated carboxylic acids such as methacrylic acid or its salts, or acrylic acid or its salts, ethylenically unsaturated phosphonic acids or their salts, ethylenically unsaturated sulfonic acids or their salts, or mixtures thereof.
 9. The method according to claim 1, wherein the water-absorbing polymer particles comprise crosslinked polymers of polyacrylic acids or their salts or polyacrylates or derivatives thereof.
 10. The method according to claim 1, wherein the inorganic solid particles from the first aqueous polymerization solution and/or from the second aqueous solution are clay platelets.
 11. The method according to claim 10, wherein the clay platelets are laponite.
 12. The method according to claim 1, wherein the concentration of inorganic solid particles in the water-absorbing polymer particle is from 0.1 to 8% by weight compared to the total weight of the water-absorbing polymer particle in dry state.
 13. The method according to claim 1, wherein the water-absorbing polymer particles have a value of Effective Capacity (EFFC) between 25 g/g and 28 g/g according to the EFFC test method.
 14. The method according to claim 1, wherein the water-absorbing polymer particles have a value of UPM (Urine Permeability Measurement) of at least 5 UPM units according to the UPM test method.
 15. The method according to claim 1, wherein the water-absorbing polymer particles are surface crosslinked.
 16. Water-absorbing polymer particles obtained by the method according to claim
 1. 17. An absorbent article comprising water-absorbing polymer particles according to claim
 16. 18. The absorbent article according to claim 17 comprising an absorbent core, wherein the absorbent core comprises one or more area(s) which is/are substantially free of absorbent material. 